Just wanted to chip in...

Flying TG from BKK to SGN was my first ever C class experience. I was flying LHR to SGN with TG, but Y was full BKK to SGN, so I paid the extra. Felt it was very extravagant and slightly "naughty" since I wasn't a business person. Felt great to be "on the carpet" at BKK check-in. (I'd overnighted.) Won't waffle on - but it was great service - but that was back in 1994.

One tip. I did once fly with AF LHR to SGN via CDG in C. I got a better fare by booking two separate legs and joining them up. I booked LHR/BKK and BKK/SGN. The reason it was cheaper seemed to be because:

1) From LHR, there are many direct flights to BKK - so flying via CDG is a pain - so AF discount LHR/BKK.

2) From LHR, there are no direct flights to SGN, but AF has one of the best connecting flights - so LHR/SGN is relatively expensive.

3) At BKK, most AF pax get off the AF flight, and for regional travellers there are more obvious BKK/SGN flights - VN, TG etc. So AF discounted this route heavily.

This reasoning was confirmed by the AF office in SGN as well as a knowledgeably FA. This was all a while back - but unless you are committed to Thai, it might be worth investigating.

I wouldn't rule out VN airlines either. They are running relatively new 777s. OK they are not shell seats, or lie flat. But I did find them nice and soft and very comfortable. Much more comfortable than my other main C long haul experience - EVA's Premium Laurel on LHR/TPE which is a angled-flat seat in a shell. I thought VN's service was excellent too , but other's have had variable experience of VN.

I have never flown TG C long haul, only Y. I've flown AF, TG and VN on BKK/HAN, BKK/SGN in C. I thought AF was the best, but nothing to choose between VN and TG.

Sorry, can't answer about the seats - my experience is too old. Don't hesitate to mix'n'match on the airlines though. Often for C the price works out pretty much the same and it can add to the variety. (eg TG on the long haul, Vietnam Airlines on the short etc).