Firstly, as already mentioned, VN operates from Gatwick, not Heathrow.
Also, looking at VN's website, some flights to Hanoi are non-stop and some via Frankfurt. They are clearly marked as such in the field marked "Stops:". This Friday's (Sept 13) has "Stops: 0", and the next three flights (Sept 17, 24 and 27) has "Stops: 1 (FRA)".
For Ho Chi Min City, most flights are non stop, but a few are marked as "Stops: 1(FRA)".
It's possible that the OP booked the flight as non-stop, but it was then switched to a one stop. The only way to check that would have been to check the reservation frequently before check in, and possibly to note the flight duration (11hr05 for the non-stop vs 13hr50 for the one stop).
I don't think there's any recourse here. The flight wasn't canceled so I can't see any issue re EU261.