Being geass'd might not really work. As mentioning in my reply to Lelouch's permissions post, I had written that Zero is super resilient and strong-willed when it comes to anyone who tries to take over his mind or compel him. To the point where even the pureblood who turned him - someone he essentially shouldn't have been ably to defy at all - couldn't control him and order him around. Lelouch can still try, though! Either way, if he does straight up ask about the whole (vampire) thing, Zero isn't going to lie (instead being very blunt). As that's simply not how he is.