I often go for 3-6 weeks in Vietnam during the summer, and I'll be going again in late august early september this year. It's true that it is the worst time of the year for northern half of the country. Hanoi is bad, and getting worse every year, it can be really suffocating in summer with 40+°C during the day, around 30 during the night, 80%+ humidity, no wind, lots of pollution. If you've never been to Vietnam, I'd say : stay in the south, the weather is actually cooler, Saigon is pretty much 32°C all year round, no typhoon...
That being said it's difficult to give advice regarding tourism in Vietnam, because areas that could have been nice last year can have been wrecked by a sudden development of local/ cheap chinese/cheap russian tourism. For example Nha Trang used to be a very nice place, now it's only nice if you're looking for underage prostitutes and cheap vodka. I used to like Hoi An as well, but it's ridiculously crowded, tons of Koreans, and people aren't as nice as they used to be. You can probably find decent beaches in remote area, but I'd say there are better countries for beaches.
I think you should look up the mountains north of Saigon, if you're looking for something else than eating cheeseburgers in a nice hotel, the areas of Buôn Ma Thuột or Pleiku I hear they aren't packed with tourists yet and the weather is nice. I went to Dalat the last time around, and Russian tourists had already reached it, that's why I'm suggesting something even more remote.
I would suggest to travel exclusively in planes in Vietnam when you can, it's cheap, safe and will save you a lot of time and disconfort. Especialy if you book in advance, Vietnam Airlines is very cheap for domestic flights.
Vietnam has the best asian food (imo) but it's not very easy to access it for tourists : a good vietnamese restaurant usually serve only one dish, or one kind of dish (for example : duck with rice, pho, ...) so you often don't even have a menu ! if you're in a restaurant with an english menu offering a large variety of dishes and there are no locals ... you're in the same vietnamese restaurant you would have home (or worse). For example something that look like this :
Is probably going to be bad.
On the other hand something like this :
Is going to be good : full of locals, small chair/table (not very confortable, i don't get it, but it's an easy way to tell the good food from the bad : the size of the chairs/tables, the smaller, the better) , only one dish.
Anyway, I'm also starting to plan my trip to Korea / Japan / Vietnam this summer, and it's a bit of a headache !