could i go zero down for VA with a 620 credit score?>
The minimum middle credit score for VA is 600 as of last month. So the answer is yes.

will my payment increase after VA cashout refinance?
If the new loan amount is higher than the initial VA loan then the new payment is likely to be higher. However, if your old rate is 4.75% or higher it is quite possible that the new payment could be lower, again, depending on how much "cash-out" you get at closing. For actual numbers give me a call at...