Do I need a re-entry permit to return to Thailand after a trip to Vietnam on a visa exemption?

In theory you can get unlimited 30 day visa exempt entries by air, AND unless you have a history of living here on back-2-back visa exempt or tourist visa entries you aren't going to have a problem stamping out on this current 30 day visa exempt entry and getting a new 30 day entry stamp when you come back. If asked you already have your proof of onward travel.

I personally wouldn't waste the money for a re-entry permit..

A reentry permit keeps your current stamp alive and doesn't cancel it. What are your travel plans? If you're staying for a long time (i.e 3 months or more, and you still have lots of time on your current stamp), it might be best to keep your current stamp alive

a 30 day visa exempt can't be prolonged with a re-entry permit. Once you leave Thailand, this "exempt" stamp becomes void. You can only then re-enter and receive a new "30 day visa exempt entry". By land borders this is limited to two per calendar year, by airport entry officially unlimited (but not intended to be overused)