Well, I think that while Mathias/Dracula is "in hiding", he has Death capture the souls of vampires killed by vampire hunters and uses the crimson stone to become more powerful. Then again, this may only happen after Lisa is killed by humans.
I, too, would like to know what Mathias does for the next 380 or so years. All we know is that in that time period he falls in love with Lisa, fathers a child (Alucard), Lisa dies, and Drac (Mathias) begins to slaughter humans. Maybe, this will be explained in manga like some stuff for CoD will be this September, though it is unlikely.