When AMD spoke about their Ryzen 4000 series APUs, they said that the APU's version of Vega contained extra optimisations that would be seen in later Radeon graphics cards.
This presumably, is part of RDNA 2's performance/watt gains. So in a way, a little RDNA is in those Vega APUs. That said, it would be nice to see the APUs go full RDNA, or RDNA 2. Sadly, RDNA 2 wasn't ready for those APUs to be designed and manufactured with that architecture.
TBH, the APUs would be great with RDNA, as they are a lot more efficient with memory bandwidth, which would be great for memory-constrained APUs.