Okay, I know this thread is dated, but it didn't migrate off the front page, so I'm going to give it a go.
Objectively: Vore is exactly what we think it all is; the swallowing or consumption of a victim, often but not always female, which has sexual or erotic overtones.
Subjectively (my take on it):
- Vore at its core elements often, and usually best, involves a large maw or orifice. This works because it invokes primal terror of being eaten alive, but in many cases the female victim is in states of scantily clad and degrees of helpless, which leads to a deep eroticism and fetishising of vore as a genre. The opposite is vore from multiple creatures; pirahna, which keeps the same erotic tropes.
- In my view, it's more a subgenre, that has crossover with other genres/subgenres...these include victim sacrifice and other common horror tropes. Cinema references include the hellmouth/esophagus scene in Poltergeist, 1982. The sarlacc; Return of the Jedi, 1983. The worm pit sacrificing scene in The Lair of the White Worm, 1988. In RoTJ, this involved Leia in her iconic metal bikini. In the other two, the intended victim was female, and wearing bra & panties, or panties. Also; Deep Blue Sea, 1999. The girl is not scantily clad, wearing a wetsuit, but the shark jaws notably clamp about her hips, coming up between her legs, with a very gratuitous gush of red, bloody water.
- My personal preference; plant vore is more terrifying and erotic. It's more alien, even Lovecraftian in feel. Besides the gnashing of teeth, plant vore is often a more extended and terrifying death, which the victim may have to endure for a long time.
- Flesh caverns/flesh caves. Appear in hentai and ryona themes; example, GIGA studios, which has crossover with tentacle bondage and rape.
- IMHO, not a subgenre of tentacle porn, as that mostly involves the sexual violation of the victim.