You don’t need a specific soft. What’s good for the goose is good for the gander. 🙂

A standard VTT soft works perfectly fine for just one player and one GM.

Now, if you’re thinking about a virtual GM, then it’s not gonna work, because I’ve known flesh and blood GMs who threw in aliens in a hill street blues-like campaign and I’m quite doubtful about the ability of a computer to surprise us with stuff like that… and I am expecting stuff like that, not bashing mobs, otherwise, I’d be looking for it in mmorpgs, if you get my drift. ;P