I think it's way too early to say either way whether something like this will happen.
1. We still need a vaccine. Yes, there's been great developments announced this week, but there's still a long way to go.
2. When a vaccine is made available, it will (or, at very least, should) be made available to older generations, NHS staff, etc first. The overlap between people who have been given access to the vaccine and people who want to go to the park at March will be small.
3. If you're requiring guests to be vaccinated, staff should be too. And then you encounter the same issue as above.
I can definitely see it happening for some events at some point in the future. I think start of season 2021 is too early for the parks to implement that. We could see similar ideas about having to show you've tested negative in the last 48hrs of your visit, for example, if regular testing with quick results (like happening in Manchester at the moment) becomes more of the norm. But then you're relying on the country's testing systems, which has been shown to not be as robust as it needs to be.
With the situation changing so much and developments moving rapidly, I don't think we can really say what the 2021 season will look like in terms of Covid-regulations until February at the earliest. There's just way too many variables at this point.