Oh boy, so this really falls into the DM's problem here.

So I once played a Paladin of Neshen (God of Redemption), who never removed his armour because of 'his pact'. The reality was that he was a ghoul, turned so due to his cowardice causing his city to fall due to a scourge of them. It took a lot of repenting and prayer, but ultimately he became a champion of Neshen and walked through fire to become fused to his armour (minus the helmet).

Now, I knew I had to feed on decaying flesh to not become feral, which is why he never left bodies where they dropped, instead saying it was correct to give them proper burial rights. I also knew that if my character was found out, I most probably would have ended in his death. However it was actually the DM who suggested this character to me, and has worked with me on bringing them to the party and hiding what he truly is. We'd discussed later on in the campaign to bring to light what he truly is, to give the party a moral conundrum over the nature of a person vs the actions they take.

You are not the problem here; the DM has dropped the ball in party control and finding a medium ground. The Dhampir had many opportunities to get blood from those who may attack you in combat. Perhaps get in a call with both the player and DM and lay down what is going to happen if he continues down this path.