CVD[p, i] is a D-1-dimensional Cartesian vector and is
transformed into
CartesianPair[CartesianMomentum[p,D], CartesianIndex[i,D]]
by FeynCalcInternal.
Overview, FVD, Pair, CartesianPair.
CVD[p, i]p^i
CVD[p - q, i](p-q)^i
FCI[CVD[p, i]] // StandardForm
(*CartesianPair[CartesianIndex[i, -1 + D], CartesianMomentum[p, -1 + D]]*)ExpandScalarProduct is used to expand momenta in
CVD
ExpandScalarProduct[CVD[p - q, i]]p^i-q^i