I read this as being a per-vampire choice, rather than per army. If not, no one will ever pick martial!
A question came up in a game recently - can you pick Unstoppable Armies for your battle tactic if you have no units to return? Endless Legions doesn't seem to require you to have a unit to return in order to roll the dice, just that normally there would be no reason to bother.
As an extension to the question above, can you roll Endless Legions if you have been tabled? Again, nothing to say you have to have a hero alive to use it or anything...
