What should I expect when re-entering Thailand on an Edu Visa after a short trip to Vietnam?

Make absolutely sure you get a re-entry permit before you leave Thailand, or you'll lose your visa as soon as you leave Thailand if you don't have one.

Ja imam ed visu vratio sam se iz vijetnama prije 5 dana i provjeravaju javi mi se privatno pa da ti kazem sta sve pitaju

Very likely they will ask you questions about where you are staying, why you are in Thailand, why you went to Vietnam, etc. They will also likely ask you some questions in Thai to test your Thai a bit.

Because an ED extension requires you to attend class in person, they will be wary of you leaving the country.

It shouldn't be an issue really, especially if it's over a weekend or there is a holiday in that time, and you can just say you went on a short holiday to visit a friend and there wasn't class during that time.

Also make sure you get a re-entry permit or you will lose your visa/extension when you stamp out of Thailand.