Vampiric domination does not persist after forcing a vampire into gaseous form but can be reasserted when the vampire returns to it’s natural state.
Domination is automatically terminated when the vampire is truly dead.
The burden of evidence for “doing something against my nature” to incur the new +2 saving throw is transactional (meaning, the player and the DM must each pose their case to the extent that one side gives up the argument to the other or the result is approved by collective acclaim), and must be include a consideration of the character’s alignment.
Post-Domination efforts do not exist in the action economy EXCEPT (a) it’s a move action to give a thrall new orders, and (b) it’s a full round action once per day to maintain the Domination through the duration of the affect (12 days for vampires unless otherwise specified)
Protection From Evil, retroactively applied to a dominated character, is temporary according to the spell description. Protection from Evil proactively applied precludes domination completely for the duration of the spell.