Well I feel like the "convince him you're not dangerous" ship already sailed, and it would have never really been Vashti's style anyways. If I've approved an option as a write-in then it's going to be a sincere choice. Take that as you will: whether you want that sincere choice to really be at the center of your motivation as a Vicar (under the more broad Ambition) or whether you'd prefer for something else to take its place instead.
Choosing one option doesn't mean it's the only motivation, though, but it's the one that you can think up on the spot while your back hurts from being punched into a printing press. Something that comes to mind easily and that you've internalized enough that you can use it to answer him readily.
Remember that as Bluff said, he could kill you at any time. He's already made an active decision not to, so try to pick more based on what you think would be best or most interesting for Vashti than what would convince Bluff.
You have plenty of time to discuss and chat about this or switch your vote, so don't hesitate to if you want.