I think you should join up with the group, but then subversively begin to destroy things from the inside out. Partially cut ropes so they'll break during storms, find a way to bore some holes in the hull of the bilge, etc.
A vampire on the open ocean should be more cautious about who he brings aboard. Wait until you're more than a day from port to spring your plan into action. Sabotage lifeboats and dinghies (or blow things up and set the sails on fire) so the vamp can't easily escape.
If your character hates slavery, why turn your back on these guys? Think of the future slaves you're abandoning to their fates. Have your character act as an agent of destruction and bring these slavers down. If he dies, at least he does so cleaving to his beliefs.