By start of 2021 or by end of 2021?
If your question is by start of 2021, there is tiny chance for that. Yes there will be covid vaccine before end of this year, but they will still finalise testings so no one will get vaccinated out of Russia and China. That is simply not enough optimism to push Bitcoin price over old ATH.
If your question is by end of 2021, then there is much bigger chance for $50k Bitcoin. There could still be some huge economic crash that would prevent it.