Don't forget that slam uses 1.5 str and should be compared with 2 handed weapons as what is available to use to do other things. I always thought of a slam as a blunt attack, such as a slam from a vampire would be using both of his hands to "axe handle" ( I think its called when you hit someone with one hand wrapped around a fist) or some sort of all out sequence of hits, one that would use multiple appendages to equate for the 1.5 str damage output. I know that a slam is a natural attack and it says any appendage could be used but I think the DM should really think about this on a creature by creature basis. A vampire obviously cannot slam with his head, how the heck would he do 1.5 damage with his head, or even a kick isn't really powerful enough IMO. For a vampire I would require the use of his hands to be part of the slam attack.