So I joined the Dawnguard, and Florentius gave me a mission to rescue Alvor at Silent Moons Camp and kill the master vampire who kidnapped him. However, when I go into the camp Alvor is there and capable of being rescued, but the master vampire doesn't spawn and a marker to kill him just hover's above the ground. This may not be related, but at the same time I also have a quest from the Companions to kill the Bandit Leader there. Solutions?

Go back to a save from before you got the rescue mission. I think the DG quest is always at this camp, so take care of the companions quest, at least to where you do what you have to do at the camp, then go get the rescue mission.

I'll try that out. Thanks.