Interesting question.
1. Lots more tourism in Thailand but VN tourism growing rapidly.
2. Lots of Viet Kieu traffic to VN from the US
3. Bigger/better airport at BKK.
4. Easier visas in Thailand for Americans.
5. Bigger economy in Thailand
6. Bigger population in VN.
7. Much lower labor costs for VN compared to TG.
8. NH investment/alliance with VN
9. TG not yet allowed to fly non-stop to the US
10. Better economics on non-stop flights from the US to BKK or SGN than to SIN.
IMHO all of the big SE Asia airports (BKK, SGN, and SIN) will continue to grow but SGN will see the fastest growth.