Can I leave Thailand to visit Vietnam and re-enter on a tourist visa if I'm from the USA?

No problem leaving and entering Thailand but you will need a visa for Vietnam.

And don’t forget to get an e-visa for Vietnam

you don't need a tourist visa, you will arrive on 45 day visa exempt, and once you leave for Vietnam, on the return you get another 45 days!

Make sure you apply for your Vietnam evisa early! They've been taking a lot longer than the website advises!

Many people confuse a visa exempt entry - where you arrive with no visa and get stamped in for 30 (currently 45 day until April 1st) with having an actual tourist visa (where you’d get stamped for 60 days).

There is literally no point in getting a re-entry permit on a visa exempt entry. You just leave and come back and get another.

Those with an actual tourist visa can get a re-entry permit at a cost of 1,000 baht. It will keep your existing “permitted to stay” date for when you enter again.

As an American you do not need a visa at all for less than 30 (may be 45 right now) days. You enter visa exempt.

No problem but you can return on visa exempt (no visa) dont need to apply again for a tourist visa, (with USA passport) you get 45 days. May be asked to show a ticket leaving within those 45. Once you leave Thailand, your original visa is history.