S4 E1 and how it applies to S5 Part 2

Theory using events from S4 and S5 spoilers ahead.

I see a lot of discussion of needing a 20 to defeat Vecna and the math to do it is 1+11+8=20. That's been floating since S4....but you don't need that.

In S4E1 Vecna shows up in Eddie's campaign after being thought that Vecna was dead. Which mirrors Steve's statement in S5E1 of he was burned to a crisp pumped full of lead. But alas he returned.

That said when we are down to Dustin and Erica v Vecna in the campaign of S4E1 Vecna only has 15 hit points left, so did they need 20, or just 15? Do we NEED 1 to take out Vecna? IE Will's access to 1's powers? Additionally combining 3 party members is not a "nat 20".