Snap Kick reads:

If you have dealt damage to your opponent this turn, whenever this attack gets a damage bonus, gain X vitality. X equals the damage bonus.

Let's say during an earlier attack in the same turn, I play an enhance which provides my next attack with a damage bonus if the current attack deals damage-- which it does. I then proceed to play Snap Kick. Does the floating damage bonus trigger the passive ability on Snap Kick? And what if I have stacked several floating bonuses?

yes, floating bonuses will work because they apply after the snap kick is played, so its effect is immediately active.