Just did a spot check on a random day in June, and it looks like all the SGN-DAD flights are flown on an A321, so at least there's an actual J cabin. After coming off EK J, just be prepared to be underwhelmed. Some of the A321s need a freshening, just be glad you get a bigger seat.
Hotelwise, if you didn't want to go all the way to District 1 (yes, it can be an annoying 45 minutes to get there), there's a Movenpick hotel (formerly Omni) that's on the main road to Dist 1, but is closer to the airport... could be the best option for a late arrival/early departure.
There really isn't much at the airport itself, and the lounge isn't worth getting there early (in fact, you might be shocked at the offerings). Unless you like self-serve instant ramen with pour-yourself boiling water from a stock pot. One contractor manages all the lounges at SGN.