Is a 645 credit score high enough to get a 200,000 home loan with my VA certificate

That score may be on the borderline, but there are other things that go into the credit decision.

Things like how much other debt do you have, what is that expense on a monthly basis, how much is your income and what does that do for your debt to income ratio.

There are so many on line lenders, why not just apply with one, and see what you could be approved for. There is no obligation that you take the loan, you can get just a pre-approval.