I am here 2 years (in HCMC) and have not had the same experience - although I am not fluent and travel 3 weeks of the month - making it hard to get fluent. Over the years I have found in every country I have been in that you have to help the person "clue in" a little....get their brains ready for what's coming so to speak.
A casual hello in Vietnamese helps the process, then you can follow with the "how are you" etc. In your particular scenario, the retail worker is more than likely a low skilled, low educated person (not always I know) and may not be at all confident speaking with older foreign people. I find the VN people very polite and respectful in general with only a few exceptions.
Lack of confidence happens with well educated people also who have studied English.....in a nutshell, help them clue-in and you will be more successful.
Cheers.