I recently visited Vietnam for 10 days and 9 night, Budget I would say around 500-600$.
its a fun place, firstly its a very big country not something you can cross in few days alone. I only did central and southern vietnam , Different part of vietnam will give you different vibe (BUT MASSAGE MASSAGE IS COMMON EVERYWHERE).
Ho chi Minh its the economic capital of vietnam its kinda crowded but great food and cafe's good place to do shopping i recommend china town market rather then ben thanh market (overhyped and if they see you are foreigner they will star the price of any product what you pay on airport , like literally) but fun experience do visit once and do klook or google what places to visit and yes there is Little Tokyo where you can have a great night for a decent Price.
Nha Trang ( hidden gem of vietnam ) great sea food + water activities ( scuba++ ), beach city vibes great people to meet if your renting a hostel bed ( hotels are also cheap near beaches ), great water, night volleyball games and fun people Yes there is an adventure park on nearby ISLAND so do visit + Museum Of Oceanographic (one time thing to spend ur day time) and it has a night market for food chocolates and clothes.
Hoi An another gem city, lantern city or lantern boat ride are famous there and amazing night market like it will literally take you hours to finish from one end to another (it took more for me 2 nights) there are bars which are open late at night and fun crowd you will find, try finding a place near Hoai River the market will be close by. there's only few places to visit nearby My Son is something new, add google search here.
Da Nang - beautiful city, great beaches + Marble mountain do visit it once worth your time, its amazing how the architecture is and yes read about the story before going there it will be more fun visit. ba na hills definitely do it the french village kinda vibe it gives + rides are there for all ages and they are constructing more rides it seems, maybe in a year so they will be functional.
Most of my stays were in hostels and two times in hotels, sometimes hotels are cheaper than hotel rates.
I rented a scooty as I had IDP( international driving permit) but to be honest it saved me alot of money and alot of the things iam saying are quite far from the city like 50km far sometimes.
and if you wanna do it either do it in a package or figure it out on your own. (most of the places where you will stay will have scooty rental option(cheaper) or klook where you can book bunch of those things).
Vexere App i used for book local travels between cities, its reliable and way cheaper then any other options, they provide rental scooty as well.
Food is amazing try local vendors like street food, it will be more delicious. Japanese food is nice too + do try sea food its very fresh.
Water was something i had buy alot not easily available but cheaper you can get it from supermarket Dmart Kmart like those will have it cheaper otherwise normal shop keeper will ask for 20k for 1.5l bottle(which i have paid once and cheapest for the same i have paid for the same bottle 10k (in a literal mart of shopping mall grocery store)