I am a 35 year old UK citizen. I currently work in Ho Chi Minh City and have a work permit/ residence card here. My girlfriend (she is Thai) works in Bangkok and we both prefer it to HCMC. Ideally, my role could transfer to Thailand at some point but that is unlikely for another year. We actually work for the same company but I have no work commitments in Thailand and she has none in Vietnam.

My intention is to fly to Bangkok on Friday afternoons allowing me to spend 3 evenings per week in Bangkok with her before flying back to HCMC on Monday mornings to commence my usual working week in Vietnam. My employers, the flight schedules and all economic considerations make this feasible, if not ideal.

My only concern is regarding immigration. I am happy to get my passport stamped with a tourist visa each weekend as I will only be staying for 3-4 days at a time (and occasionally for around a week or so when public holidays and annual leave allow). At what point, if any, might this be an issue if Thai immigration sees that I am coming into the country every week? Or is there another visa option that would be better suited to this particular set of circumstances?

Any opinions or advice appreciated.

Immigration will not be too concerend. Their only question could be if you are working or not, for which you can show your Vietnamese work permit/residency card.

There is no other visa option for you anyway, untill you get married to your Thai girlfriend. Coming and going with a visa exempt entry wil be fine,