First let me say thank you to all on this site who are willing to help confused souls such as myself. I am a US citizen and am already ticketed to arrive in Bangkok on December 29th and depart March 27th. I spoke to the Thai Embassy here in DC who advised getting a 60 day Tourist Visa and then a 30 day extension once in Thailand. My first question is whether there should be any problem boarding here given that I only have a 60 day visa for a 90 day period or will the airline only be concerrned with my ability to enter? The other issue is I am considering a side trip of a week to ten days to Viet Nam during my stay. I wasn't going to book it until in Thailand but am somewhat concerned that trip could complicate my Thai visa extension to the point it wouldn't be worth the hassle. Lastly I did spend 21 days in Bangkok in October, entering without a visa. Not sure if that complicates matters but thought I should mention it. Am I worrying for nothing on all this? Any help or advice would be much appreciated!

No problems at all

You have a visa so no issues with the airlines ....

no problem with a side trip .. you can either pay for a re-entry permit to keep your visa alive or just get a visa exempt entry coming back from Vietnam

Yes. A Re Entry Permit would keep your stay alive (1,000 Baht)

Or go to Vietnam and return with a 30 day entry stamp.

It all depends when you go. Up to you.