I've seen pictures of an example of a post-type vise with a clamp on base; so they're out there, I guess, but none has swum into my net yet (still waiting, along with a baby bullet, Parker, Athol, etc.)
Along that line, a local guy did a sale of his crowded garage, and he had the cutest little leg vise I've ever seen. I don't have space, or it would have followed me home.