I forgot to state the quarterly advance payments I made during 2020 of estimated state tax for 2020 in my 2020 VA tax return. Shall I file an amended VA return?

this is something VA is likely to catch. so if your due a refund, with those payments included, wait for the refund to see if it's correct. if VA doesn't catch it then you're likely to get a bill which then you can provide copies of the payments and whatever other documents it wants - such as bank statements where the payments cleared.