Assuming you accept that the original belt occupied the most appropriate slot, the vest should cost 1.5x the amount of the belt, according to the 'Body Slot Affinities' section of the DMG (/SRD).

I seem to recall that back in 3.5's heyday, a monk's vest was generally an acceptable substitute without needing to up it's cost.

That's up to the GM to decide, but if you look at the link I posted, there is quite a reasonable argument in that direction.

Is there a rule that specifically says that? I dunno, playing safe and RAW is always a plus.

You'll not find any rule so clearly stating that. The best you'll get, in my estimation, is the link I posted above, which lists the 'vest' slot as having an affinity for class feature improvement.

sorry wrong tread.

Question:

Can warforged use their slam attack as a 'weapon' for the purposes of being a weapon familiar?

If not, what are the best weapons one could take to use as one? I think a one-handed one, as it gives a +3 bonus to UMD.

Upgrade your hands into an actual weapon graft, I it's from Magic of Eberron or something. Yes, now your hands have personalities separate from your own.

Okay. Hmm... With a docent, a weapon familiar, a normal familiar (Gestalt with Wizard), my warforged is going to have three pains in the ass to worry about.

Be a Fleshwarper. Because all you want is to be a real boy. Or you could DO the normal way and just spam Extra Familiar, Improved Familiar and forge yourself some robot dogs and birds.

If you want multiattacks, wildshape ranger named dinobot.