I've been several times, with my TGF, as I have a UK PP I also didn't need a visa as I got 15 days on arrival, which was adequate for our plans. I've flown BKK - HaNoi, BKK - HCMC, and BKK- Danang. Danang is my favorite place in VN, less crowded & hectic than HaNoi and HCMC, very nice beach, lots of nice seafood restaurants, and as you're interested in the Vietnam War sites the best are in the central region of VN. We took the train from Hue to Dong Ha and from there found a tour guide who took us to Khe Sahn, the Rockpile, the site of the Special Forces Camp at Lang Vei, a VC tunnel site, plus a few other war era sites. Our last visit was pre-pandemic, if I was to go again to VN I'd fly to Danang and hang out there and Hoi An, maybe go to Khe Sahn again.