I've lurked these forums since day 1 of GiantBomb and I've never really felt like posting anything for all these years since I came across your post
I've been all over the world - from Switzerland to Kuwait to China, but Vietnam stands above all else for me. Its a wonderfully colorful country with awesome sites and experiences
Now when I've gone I've had the advantage of being able to stay there with family of my good friend (he was born there) or with friends I know that live over there. As such, I really have never stayed in a hotel and I've been able to aviod the cheap touristy crap that others might get stuck in. As interesting as it is to be hassled by shirt peddlers in Ben Thanh market with all the other tourists - the true charm of VN comes across by wandering the streets of Da Nang, enjoying the beautiful flower groves in Da Lat, or just spending a lazy evening in the village of Dong Thap in the Mekong Delta.
Of course this isnt to blow off cities such as Sai Gon with its countless things to do any given day or going to Hue to see the old imperial city (even though the government has left it in a sorry state.) I would say also go to Nha Trang - even though its beaches tend to be more cigarette butts than sand sometimes, go to Vinpearl Land if you want to see a vietnamese take on a amusement park/resort, just to say you have...
I would say avoid places like Hoi An, which may be a beautiful city, is almost nothing but resorts and shops created to sell trinkets to westerners.
Unfortunately in the times I've been over there I have yet to see the top half of the country, so I cannot speak for places like Ha Noi or Ha Long Bay - even though they look wonderful
Of course I've gone to most of these places with the safety net of having someone that at least speaks the language fluently, so choosing where to go should be determined by how adventurous you are, but for the most part use common sense about which streets you go down and at what time of the day and you should be fine. VN is a fairly safe country, but pick pocketing is of course a big thing over there, and I'm going to assume you are not ethnically Vietnamese so you are going to stand out over there and be an obvious mark for people looking for a new phone, camera or wallet.
I hope this rambling post helps you get a little inspiration for your own trip to the far side of the world