I would say yes because you are leaving over 60 days which is all you are allowed on your visa exempt. The airline may not let you board. So buy a ticket via onward (on the internet). It will be super cheap and your airline will accept it. Once in Thailand sort out the Vietnam trip. Peace of mind.
we were there a year ago but I haven't read the law has changed. Our hearts were in our throats but it was fine. No smiles from the girl behind the desk, very serious
Krabi was easy office to negotiate. If you don't have a passport photo they will take one. We couldn't get a tm30 from our hotel as it had been filed by them electronically but we took a business card and if I remember correctly they called the manager to confirm. It is out of the way - can't walk anywhere from there
Use an onward ticket if you are staying more than 30 days. They are cheap last year$15 and it saves so much worry. But if you plan on leaving Thailand to go to another country within 30 days that ticket is fine.