I may be remembering wrong (old age and all) but I specifically remember them being released at the same time because I remember people remarking at the fact that Valve would allow Vampires to be released so close to HL2. At most, it was a week after HL2's release, which still puts Vampires' release it much sooner than December or January.
Edit: my join date on a VtMB message board was December 1, 2004. Before that there was a different "unofficial" message board that I was a member of for atleast 3 weeks to a month. So based on that, Bloodlines was released, at the latest, about late October, early November here in NA.