That's good, and as you already posted, you have a confirmed onward flight to Vietnam in March.
I remember a news article quoting an immigration commander saying that proof of funds does not have to be in cash. If the IO wants to refuse you entry, he will have to play another card, for example suspicion that you would work illegally in Thailand. That would be section 12(3) but I have read about this one being pulled only once or twice in the 14 years I have been reading this forum.
Wishing you good luck.