Super nice, I like Hanoi more than HCM City but HCM is still nice.

Definitely get to the Hanoi Old Quarter if you can and absolutely seek out the propaganda poster reprinting shop if it's still there. Also if you like beer then get stuck into Bia Hoi while you're in the Old Quarter.

The War Remnants Museum is worth visiting in HCM City.

If you have the opportunity and you like hiking, head up north to the Sa Pa mountains and spend some time with the Hmong villages. It's very picturesque and the Hmong people are super lovely, even by the already-high standards that Viet people set.

I wish I spent more time in Ha Long Bay. The islands around Cat Hai are absolutely stunning if you like nature and physical activity (I kayaked around Cat Ba island and surrounds. So good!) but Ha Long Bay also has nightlife and fancier activities if you prefer drinking and cruises and stuff too.

I'd recommend doing your research about what's out there and it's handy to have a few conversational viet phrases under your belt when you go because it's not necessarily as catered to English-speaking tourists, depending on where you find yourself. There's a lot to see and do and Vietnam offers so much. Try to think about whether you want to do partying/history/culture/food/recreation/sightseeing/shopping/relaxing/adventuring and what sort of mix of these activities you want to do, then build your itinerary around that.

Oh also get ready for SE Asia and crossing the roads, if you aren't already prepared for it lmao. Maybe watch a few videos of people walking across large, busy streets in a place like Delhi or Jakarta on YT to get an understanding of how to do it because if you're not used to how to traverse a road on foot in Asia then you will want to go in prepared because it can be a culture shock for mayos.

At one point I was toying with the idea of moving there longer term. I like Vietnam more than Indonesia, Japan, and Hong Kong (never got to the mainland unfortunately) but that's a very subjective opinion.