FVD[p, mu] is the D-dimensional vector p with Lorentz index mu.
FVD[p, \[Mu]]p^{\mu }
FVD[p - q, \[Mu]](p-q)^{\mu }
FVD[p, \[Mu]] // StandardForm
(*FVD[p, \[Mu]]*)FCI[FVD[p, \[Mu]]] // StandardForm
(*Pair[LorentzIndex[\[Mu], D], Momentum[p, D]]*)There is no special function to expand momenta in FVD.
ex = ExpandScalarProduct[FVD[p - q, \[Mu]]]p^{\mu }-q^{\mu }
ex // StandardForm
(*Pair[LorentzIndex[\[Mu], D], Momentum[p, D]] - Pair[LorentzIndex[\[Mu], D], Momentum[q, D]]*)