You SHOULD be ok. When I was in Thailand earlier this year, after a couple of weeks I went to Cambodia for a few days, came to Thailand for a couple of more weeks, then went to Vietnam for a few days, and then came back to Thailand. On that last entry from Vietnam the IO did ask me how long I planned to stay in Thailand "this time." I was able to say that I only had 11 more days and then I'm going home to America. I suspect if I had planned to stay longer he would've given me a warning. He didn't ask to see my ticket back to the US, but, if you haven't already done so, you should buy your ticket to Vietnam and be ready to show them.