Suppose a vampire is two-weapon fighting. Does it have to drop a weapon to use its slam attack or can it use other body parts - kicking or elbow striking for example - to attack with a slam? I ask this because usually a creature must forgo one natural attack for each weapon clutched in a limb, but slam attacks are a little more nebulous.
You probably cannot make attacks with two weapons and slam as well with a vampire.
The rules are a little unclear what exactly constitutes a slam attack because it is the Natural Attack that is granted to creatures with no appendages. However it also seems to be the Natural Attack granted to creatures with arms but not Claws.
This is supported by the Natural Attacks description which includes
...although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam).
Of course, they had to include that word "often" so you can rule that's not the case. I don't recommend using the Slam when you're also using multiple weapons, both for narrative and balance concerns.