YES you did ask this question before.
IF you are traveling as you say...
There's NO need for you to get a Thai Visa... TOURIST or Multiply entry tourist visa.
You can ENTER Thailand VISA EXEMPT... Getting you stamped in for 45 days...
And as you are leaving Thailand to Vietnam in 29 days...
AGAIN when you return to Thailand from Vietnam in 29 days..
You will be stamped in for 45 days VISA EXEMPT.
Just have the copy of Each ticket for your flights ...to Thailand, to Vietnam...back to THAILAND AND your Return ticket departing Thailand for your home country to show Qatar Airways...
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