Will my non-immigrant OA visa expire if I travel to Vietnam for 3 months?

Remember to update the insurance as your extension if any is only valid until the insurance expires

You will need to enter Thailand before the expiry date/enter before/valid until date written on the Visa to get another one year stamp. If you plan on leaving Thailand during that second year you will need to get a reentry permit to keep your stamp alive. If you're leaving on the 10th month and coming back 3 months later you can kiss your OA goodbye.