From my Google Flights search, there appeared to be two options for non-stop flights from Bangkok to Da Nang, Vietnam: Bangkok Airways and Thai VietJet Air. Honestly, I tried to buy tickets from Bangkok Airways first, but their site would not accept my credit card. But I consider this to be failing up, as I have long dreamed of being called a SKYBOSS.
Flight VZ960
BKK - DAD
Departure: 10:50
Arrival: 12:30
Seat: 1C
We had an inauspicious start to our SKYBOSS experience, as the priority check-in lane was blocked.
Really cool uniforms
No matter, we proceeded to the normal check-in lane and were promptly issued boarding passes and provided information about the lounge and other SKYBOSS benefits.
Missing from the list of SKYBOSS privileges is access to the priority security/immigration lane. Perhaps we should have attempted to access it, as here our experience became considerably less SKYBOSSY--we waited for about 25 minutes.
The SKYBOSS lounge is the Coral Lounge in the D concourse, at the end of a long row of lounges. This is a surprisingly good contract lounge, with a tasty breakfast spread, what appeared to be a full bar, and massage services. The downside was that it was about as far as possible from our gate in Concourse E (almost a twenty-minute walk). Perhaps Thai VietJet generally leaves from closer gates, but if not, I am sure that one of the many other Priority Pass lounges would be a better option.
We arrived at the gate just before boarding started. Boarding began with a gate agent holding a SKYBOSS sign aloft. We were the only two passengers who identified ourselves. The gate agent asked us to wait, as the van had not arrived yet.
After about five minutes, we were beckoned to approach. We were apparently the only two SKYBOSSES on the flight. The agent scanned our boarding passes and escorted us to our ride. A fellow foreigner took umbrage at us cutting the line.
We had a pleasant ten-minute drive to what I believe was an A319, where we were the first to board.
We were seated in the bulkhead, which had a comfortable amount of legroom. It seems as if the first three rows are designated SKYBOSS seats, as the rows were otherwise empty. However, I'd assume that Thai VietJet would be more than happy to move the (invisible) curtain back if demand existed, as there appeared to be no difference between the seats. Also, no seats in the first three rows are blocked, so it's conceivable that you could be sitting next to two other SKYBOSSES.

Once seated, the purser introduced himself, thanked us for choosing SKYBOSS, and gave us a pre-departure cup of water and moist towelette, both SKYBOSS-branded.
We took off on time, and the purser presented us with SkyCafe menus. My understanding is that you can order as much as you want from the menu. I ordered a curry, which was better than anything I've had on a US airline in recent memory, and subsequently snacked on a bag of mango crisps.
We soon began our descent into Da Nang. We landed on time, disembarked via a jetbridge, and were through immigration within ten minutes--just in time to grab our bags off the carousel (the first ones out).
All in all, we had a very pleasant experience on Thai VietJet. I wouldn’t hesitate to fly them again, but I wouldn't pay extra unless I had a front row seat.