is a visa needed?

thanks you for your recent reply regarding whether we need a visa. Thankyou for advising that we wouldnt need a visa for the first trip. However, I still wasn't quite clear as to whether we would need a visa when we return from china by train to Vietnam in November . we are british citizens and fly from london to Hanoi on 5/10/16, then catch the train to china for 5 weeks on 17/10/16 before catching the train back to Vietnam around the 25/11/16 to catch our flight back to the uk from hanoi on 30/11/16. If our second visit to Vietnam is more than 30 days from our previous visit, and both visits are under 15 days, would we need a visa for this 2nd visit or are both visits exempt from needing visas?
many thanks for your help