Can the Vagabond Mystic be used during this action to heal
that vampire? If so, can he heal them back to full capacity,
or only "starting life" (the amount of blood they had when they
played the Trick)?
If he gets into combat or is otherwise damaged (can't think of how
right off the top of my head), can he be saved with Left for Dead?
If so, does Left for Dead drop off at the end of the action or
stay on him, preventing him from untapping, as for a real ally?
In combat he can be so saved by a Methuselah whose turn it is not, sure.
It won't work on the "otherwise damaged" bit, by card text on LfD.
> If so, does Left for Dead drop off at the end of the action or
> stay on him, preventing him from untapping, as for a real ally?
It stays on him, but has no effect after the action is finished, since
it only works on allies.