I fly to Thailand in December with a tourist visa that I will extend 30 days in Bangkok. My idea was to do the same as you, go to Ho Chi Minh and apply for another tourist visa, spending the minimum time in Vietnam. I didn't think there would be any problem. Perhaps a visa run with Laos using an agency is a safer option.
thanks a lot. Regarding proof of funds, do they ask to show bank statements for the 6 months prior applying having at least 20k baht in the account? Or the 20k is for the same month you are applying?