From US Census info about Vermont
Population 625,741
% under 18= 18.3%= 114,511
Voting age population = 625,741-114,511=511,230 derived
Vermont Elections data 2020
Registered Voters. 506,312
Votes. 370,968
Turnout. 73.27%
Absentee votes. 280,455
Absentee/votes. 75.60%
I do not know where the 520k figure came from. It may be that % under 18 dropped from 18.3% to 17%. Idk.
There are 2 changes from 2016, a rise in registration % (506k/511k =99%) and a rise in turnout from 66% to 73%. These are related to one another.
The turnout may be affected by mailed ballots, but the rise in registration will persist. In 2022, the turnout could go back to 60%, but it will be 60% of a larger number of registered voters than 2018 was.
Washington Post had a story about Georgia that said they changed their registration procedure in 2016, making it the default when renewing drivers license. You can opt out, but you would have to check an extra box. Their registered voters went from 78% of eligible voters in 2016 to 98% of eligible voters in 2020. This is comparable to Vermont.
I hope this helps clarify. If not, never mind.