If the P.C.s are hot on the trail of a villain it just doesn't make sense for the group to stop because the vampires can't continue.
If all the Vampires can't earthmeld if they are out in the wilderness vampires will always have to turn back early to reach that shelter they need.
Unless the villains are idiots it really doesn't make any sense for them not to attack when a significant portion of the group is mostly helpless.
I'm not saying it will happen all the time but enough to be a royal pain for the Vampire P.C.s
I've been in a game that mixed creatures ( granted OWoD) and all those problems and more came up until we decided to just go to everybody playing Vampires
The ghoul idea is a good one although obviously Ghouls are significantly weaker than any creatures