Now, although my training has been in zoology, rather than botany, I find a few problems with the concept.
It is true that small rodents and lizards have occasionally fallen victim to the larger species of pitcher plants. However, these are essentially pit-fall traps; the prey is lured into a deep pool of digestive juices and drowned. The man-eating plants of fiction and legend almost invariably involve the movement of jaws or tendrils. In plants, such movement is produced by changes in hydrostatic pressure, and there will be a size limit at which such a mechanism becomes uncompetitive with the nerves and muscles of the prey. Furthermore, although the ability of higher animals to detect and avoid danger is not unlimited, it is bound to be greater than a plant's ability to choose the right ambush site on which to grow - especially since the advantage of the site will change with the growth of the plant.
Then again, it is far from obvious what advantage a plant might gain from such a lifestyle, or how it would survive in the absence of prey. One thing is certain: any advantage is likely to be greater for the smaller species - those closest in size to the known insectivorous plants. There should be no discontinuity in size; a plant big enough to eat a man must have evolved from one big enough to eat a dog, and the latter from something big enough to eat a rat. Indeed, the younger versions of the same plant must consume a set of smaller prey. And the smaller plants must greatly outnumber the larger ones, just as jackals outnumber lions.
In addition, cryptobotany should, in theory, be an easier field of research than cryptozoology. Plants, after all, cannot run away and hide when you come looking for them. And these plants, by definition, must grow along animal trails. Their presence must be notorious to the inhabitants of the area. Yet, for some strange reason, every traveller describes a different type of plant to the others, and equally mysteriously, there is never any mention of smaller varieties in the neighbourhood.
For these reasons, you will gather that I have grave misgivings about the following reports. But they have, nevertheless, never been disproved. Besides, why shouldn't you be allowed to decide for yourself?
His reputation took a turn for the worse with his 1932-3 expedition to Chiapas, because his description of the Lacandon Indians bears little resemblance to what qualified ethnologists have to say. In his initial account, he tells of his party's advance into the dense jungle, and then:
Suddenly I saw Domingo, the leader of the guides, standing before an enormous plant and making gestures for me to go to him. I wondered what could be the matter.He gave a slightly different version the following year, in his book, In Quest of Lost Worlds:
I soon saw; the plant had just captured a bird! The poor creature had alighted on one of the leaves, which had promptly closed, its thorns penetrating the body of the little victim, which endeavoured vainly to escape, screaming meanwhile in agony and terror.
"Plante vampire!" explained Domingo, a cruel smile spreading over his face.
Reference: ' Midst Pygmies and Pyramids' by Comte Byron Khun De Prorok, The Wide World, no. 436 (August 1934), pp 294 - 303, at p 295.
Suddenly, Domingo came to a halt and called us over. Schmelling [the doctor], too, thought it would be interesting. I had my first glimpse of the vampire plant which, two or three days earlier, had trapped a bright little bird on its treacherous leaf, and now was in the process of taking is meal.It is noteworthy that we are provided with hardly any details of the plant itself. What is the truth? Has he just inserted a completely imaginary event into the narrative without any fanfare? Or has he written in the first people a local legend which really belongs in the third person? Or did he really come across a bird which had accidentally got itself impaled on some thorns, and embellished the account? Or perhaps it really happened - and no-one has ever seen that species since. At least it can be said that a plant which ambushes birds is more believable than one which preys on mammals, which tend to use defined runways.
The 'snake-tree' is described in a newspaper paragraph as found on an outlying spur of the Sierra Madre, in Mexico. It has movable branches (by which I suppose, is meant sensitive branches) of a 'slimy, snaky appearance', which seized a bird that incautiously alighted on them, the bird being drawn down till the traveller lost sight of it. Where did the bird go to? Latterly it fell to the ground, flattened out, the earth being covered with bones and feathers, the débris, no doubt, of former captures. The adventurous traveller touched one of the branches of the tree. It closed upon his hand with such force as to tear the skin when he wrenched it away. He then fed the tree with chickens, and the tree absorbed their blood by means of the suckers (like those of the octopus) with which it branches were covered. (Quoted in Searching for Hidden Animals  by Roy P. Mackal, pp 263 -4)At first, you might think this is an exception to the rule that no two travellers describe the same species of carnivorous plant. However, the vampire plant relied on thorny leaves, whereas this one relies on suckered branches. But an anonymous traveller reported in an unnamed newspaper is not much to go on.
The contributor of the following account was a Mortimer Sheppard, who claimed he was present in the Stranger's Club at Colon, Panama when a Chinese member, Lin Wah, recounted what happened to his cousin, Wing Lee in southwestern Costa Rica.
As he explained it, the place was full of legends of conquistador and pirate treasure hidden in the jungle centuries before, the site known only to the local aborigines. In this case, Wing Lee felt one of the stories had the ring of truth about it, so he prevailed on two other Chinese, Chang and Wong to help him search for a particular cave where the gold was said to be found. Hiring a group of "beachcombers and outcasts" as peon carriers, they headed into the interior, finally reaching the end of the trail in dense jungle.
On the third morning after entering the jungle, they heard a scream from the lead man, and found him in the grip of what they at first took to be a giant snake, but which was actually a vine. After being cut free, he explained that he had been hacking his way through the undergrowth when a gigantic butterfly, with a wing span of 18 inches, and completely black, fluttered overhead. As he watched, he was encircled by the monstrous vine. Less than an hour later, another carrier was snatched by another vine. Again, the huge, black butterflies. At this, all but three of the carriers refused to go on. Two hundred yards farther on, they were met with the sight of a large puma hanging six or seven feet above the ground, covered with huge, black butterflies feeding on its rotting flesh.
That night, they set up camp, with the Chinese keeping watch, Chang taking first duty. About midnight, Wing Lee awoke. Chang was missing, as was one of the peons. He alerted Wong, but the group decided it was too dangerous to venture into the undergrowth in the darkness. Come the daylight, they found Chang's body less than fifty yards from camp, with numerous black butterflies hovering above it. It is interesting to note that there was no mention of his crying out when he was captured. In any case, they cut him down and covered his body with loose earth and leaves.
Nevertheless, Wong insisted on going on. He climbed to the top of a tall ceiba tree to gain a view of the open country. From there, he gave a shout that he could see the cave. Unfortunately, at that point, he waved his arm - and lost his grip, falling eighty feet to his death. The last two peons fled.
Lee tried to make it to the cave, hacking away at every liana he saw.
Like the so-called 'sensitive plant', which will close up when barely touched by one's finger, the dangling ends of the vine weaved and curled, ready to constrict should their sensitive tips come into contact with live flesh.To cut a long story short, Lee made it back home alive - but only just. And he never did find the treasure.
Reference: 'Hangman Vines and Carrion Butterflies' by Mortimer Sheppard, The Wide World 713, pp 100 - 103 (Jan 1958 Australia and NZ, Dec 1957 UK)
According to Sheppard, a botanist in the audience at the Stranger's Club made a few pertinent comments. He said he had often come across the rotting carcass of an animal covered with the great blue morpho butterflies. Black butterflies feeding on carrion is therefore not impossible, but he found it hard to see what the vines got out of it - except, possibly, fertiliser.
I can add a few comments of my own. Firstly, despite what you may have seen in the Tarzan movies, vines grown upwards, not downwards. Secondly, there are a number of species of butterflies and moths which are black in colour, but they are not common. Also, the wing spans of the largest moths and butterflies come to 12 inches so, even if the dimensions of the 'carrion butterflies' were exaggerated (and no-one had the chance to measure them), they are still at the outer limit of size.
Also, if Wing Lee told Lin Wah, who told Mortimer Sheppard, who told us, this makes it third hand. You can draw your own conclusions.