I plan to go to Saigon for a week on Monday and my 60-day visa will expire while I'm in Vietnam. When I'm back in BKK, am I gonna have 60-day granted or should I go to immigration asap to pay the fees and have 30 days additional ?
TLDR : Answer Summary
When you leave Thailand to visit Vietnam, your current visa will be canceled upon exit. Upon returning to Thailand, you will receive a free 60-day visa exempt entry stamp, depending on your nationality. This entry stamp is not a visa but allows you to stay for 60 days. You can extend this period for an additional 30 days at an immigration office for a fee of 1900 baht.