I spent most of November in Vietnam, (Saigon, Phu Quoc, Hanoi, Halong, Hue, Hoi An, with stop offs at others in between)
I hate sand, so probably not best placed to give advice re a beach holiday, but for what it’s worth, I had one of the best few weeks of my life there in November. We just arrived, met some mates who live in Saigon and took it from there, bought/rented motorbikes a few times and lots of flights hours before we were due to depart, basically just made it up as we went along. I’m not sure I’d travel all that way for a “cheap” beach holiday though, especially when neighbouring counties do that better, but you can’t beat Vietnam for an adventure….
Incidentally, of those beach “resorts” that we saw (eg Nha Trang, Da Nang and to some extent parts of Phu Quoc), we thought looked like someone had dumped Magaluf in Vietnam and then gave free tickets to anyone with a Russian passport. My idea of hell, but others seemed to love it. Just didn’t seem like Vietnam to me)