Is there anyway to follow the vampire side and get the crossbow upgrades or find one on a dead dg soldier. If not can you get vamp royal robes while doing the danwguard quest.

Yes, you can get royal vampire armor either by finding one in between two coffins by the secret entrance to the soul cairn or by taking it off harkon's body.

You can get the royal vampire armor from the underground section of Volkihar castle while on your way to the soul cairn. The crossbow upgrades are dawnguard only though.