I'd add to Viet Tran's suggestions:
1) Hanoi - The Old Quarter, and a walk around Hoan Kiem lake. I spent three weeks there on my last visit, could have stayed longer! So much to photograph!
2) Hoi An - A photographer's dream, even with the crowds. You just have to wait a bit longer to get the shot. Get up at dawn, go to market, walk the streets as the old part of city is waking up. Lots of good cafes and restaurants. Even though I've been there several times I spent three weeks there on my last visit, and again, could have stayed longer!
3) Get off the beaten track in the Mekong Delta. Take the local buses, with the locals, rather than a tourist coach, and see more of the real Vietnam. Lots of possible routes, eg, Saigon - My Tho - Ben Tre (by ferry, there and back - I spent a couple of days there and only saw two other tourists, in the distance) - back to My Tho - Vinh Long - Can Tho (backwaters, Cai Rang floating market, Ho Chi Minh statue!) - Chau Doc (take a boat up the Mekong River to Phnom Penh in Cambodia) - or, head back to Saigon via Sa Dec. Very cheap fares, flag the bus down and jump on - note that sometimes they don't actually stop, just slow down, so you are climbing onto a moving bus - travel light!
4) Halong Bay? - One night is probably enough. All the boats go to the same places. Problems with pollution (https://www.euronews.com/travel/2023/05/29/it-makes-you-sad-tourists-and-fishermen-leave-vietnams-ha-long-bay-covered-in-trash). I did it nearly twenty years ago when there were fewer boats, and none of them were painted white! They were timber, junk-style. They still are inside, but the Vietnamese government decreed that all tour boats had to be painted white, and thereby destroyed part of the attraction of going there. I splashed out on a luxury boat, the Dragon's Pearl - great cabin and excellent food. I think it sank later, not too sure. I did it one time, but now, I wouldn't bother going there again, for the aforementioned reasons. If you go, be very aware of safety on the water. Over the years there have been several boat sinkings, and fires, and tourist deaths - Google "halong bay tourist boat sinkings".
There are millions of photos of Vietnam on the internet, but feel free to check out my galleries for inspiration! I often put the location in the description, but if you want detailed info on locations of shots, please ask me.