To get the vertex pos do this:

float $fVtxPos[] = `xform -q -ws -t ($sObjName + ".vtx[0]")`;

I think this would return all vertex positions associated to a face...

float $fVtxPos[] = `xform -q -ws -t ($sObjName + ".f[0]")`;

there might be an easier way for getting the face center. not sure...