I found a few threads in the search but they ended with disagreements on rules. My character is a vampire which gains a slam attack from the template and is using a 2 handed weapon with the power attack feat.
- Does my slam attack get the full bonus of the power attack feat?
- Does my slam attack get my full strength bonus?
- Do I suffer two weapon fighting penalties or is my slam attack at a -5 for multi weapon fighting with no penalty to my iterative attack?
Creatures with natural attacks and attacks made with weapons can use both as part of a full attack action (although often a creature must forgo one natural attack for each weapon clutched in that limb, be it a claw, tentacle, or slam). Such creatures attack with their weapons normally but treat all of their available natural attacks as secondary attacks during that attack, regardless of the attack’s original type.
Abraham's answer is the best one, but I'll clarify a bit more.
Does my slam attack get the full bonus of the power attack feat?
It gets the full bonus of a secondary natural attack (+1 damage for each -1 to attack). This is the same as with off-hand weapons.
Does my slam attack get my full strength bonus?
No, secondary natural attacks only use half strength.
Do I suffer two weapon fighting penalties or is my slam attack at a -5 for multi weapon fighting with no penalty to my iterative attack?
The latter, except that the -5 is because of attacking with a secondary natural weapon, not because of fighting with multiple weapons.