Question: When making a roll on Majesty to Awe people, is a Vampire permitted to make use of the +2 (or +4) social bonus for the merit of being exceptionally attractive?

Answer: 
Yes.
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Unless explicitly forbidden, Merits (like Striking Looks) do apply to Discipline rolls.
I've gone with the rule of thumb that effects can be stacked unless the rules explicitly say they don't. I don't see anything in the rules saying they don't. Logically it makes sense that beauty would play a role in successfully Awing someone. If I were running the game, I'd allow it.