Pre 2021 cars will have their Co2 emissions converted into a WLTP equivalent I think - theres a formula published for it. So I'd imagine you'll be using that for paying VRT, but then paying tax on either the CC or Co2 based tax system - which is similar to how an 2000 car would currently pay VRT based on its Co2 but then be taxed on CC. Then all 2021 registered cars will be on WLTP VRT and Tax.