If you are eligible for visa exempt entry (45 days until April 1) then you need nothing special for Thailand. Both times are under the 30 day typical visa exempt entry. As long as you have your exit ticket to Vietnam in hand - you are fine as you are
Happened to me also in December. Went back to the airport and it was a quick fix. It did require an explanation when I got my extension in Hua hin. But easy to follow the stamp trail