How are you planning on supporting yourself? Showing this money is how the Thai government ensures you're not destitute and then become their problem when you're broke and trapped.
That's about 60 days within the last year. That's not excessive.
Just make sure when you arrive the first time you have your flight to Vietnam ready to show if they ask about when you are leaving. And then the 2nd time when you return from Vietnam, make sure you have your flight back to South Africa (or wherever it is you are traveling to next) available in the event they ask that time.