The stated ability is an excellent form of camouflage, and it evolved b/c it helps a vampire to both hunt prey, and escape from predators.
The bigger issue is this ability is very hard to do for any reasonably complex organism, especially for one with bones. Bones take forever to form naturally, so de-constructing and re-forming them will likely take a long long time, and leave the vampire very vulnerable during that time.
Even with soft tissues, returning to normal form from a puddle means that every cell in the body knows where to go, and has a way of getting there.
Alternatively, Vampires are colonial organisms. Every cell can play any function in the body, and it can tell what it is supposed to do. Or maybe a small % of cells specialize in serving as nerve cells, and supervise other cells assembling themselves back into upright shape.
See this Q for more ideas:
Sentient colonial organisms?