I've been to a few of these countries. My impressions -
Cambodia - This sounds so condescending, but..they are very pitiable. Everyone uses the US dollar instead of the local currency there. Many merchants there speak Vietnamese because Vietnamese are so important to the local economy. There is a lack of strong national identity and/or pride. The rural landscape is admittedly beautiful, and Angkor Wat is well worth going to.
Indonesia - I've been to Jakarta and Bali. Jakarta has the traffic of Los Angeles, no public transportation, and nothing to do. Don't go. You can *feel* the touristyness of Bali all the time, seems like every shop and building there has something to do with tourism, there's hardly any authenticity when it seems like every place is marketed towards Australians. The people are typically very nice and friendly though, although stay away from the obvious scams (don't exchange money at anything that isn't an official bank, for example)
Philippines - Haven't been here in years, so don't have much to say about it. I recall it being Catholic - churches and cathedrals everywhere. Most people speak English and definitely lean on the friendlier and hospitable side of Southeast Asia.
Singapore - If you've never traveled outside of the West before this is definitely the safest place to travel. Everyone speaks English - even that old Chinese lady you wouldn't expect to, it's completely clean and safe, big malls, urban squares, ritzy buildings for you to have fun. If you're looking for cultural experiences beyond that though, it's pretty lacking.
Thailand - Also haven't been here for over 10 years. Thailand seems to be very well known among Western tourists, so I don't have much to contribute except their KFC tastes nothing like American KFC!
Vietnam - Lol. The motherland. Now...I don't want to discourage anyone from traveling to Vietnam, but....let's just say, you could generally describe the other countries above as friendly and hospitable. Not Vietnam. When Vietnamese people see a tourist, their first thought is "how can I make money off this person?" Cab drivers will rig their meters, restaurants will have two menus - one for the local and one for the foreigner -, hotels will be deceitful about the pricing, hooligans on mopeds will snatch your purse and before you know it they are gone. Don't take it too personally though, Vietnamese people don't even trust each other. Hustling others is the way to get ahead in Vietnam.
Now that I've scared everyone off, on the plus side objectively I'd say there are some great places worth seeing in Vietnam. Ha Long Bay is worth it. Hoi An is interesting for the authentic, preserved city that is like a window into French colonial times. There are cool imperial palaces, warm beaches, and Buddhist temples to go to. Unfortunately most of the Vietnam War trails and tunnels have been rebuilt or were constructed after the war for tourists, so keep that in mind if you decide to check 'em out. It's an interesting place, and definitely one that won't feel too touristy. You just gotta be on guard that's all - most Vietnamese people have no ill will towards outsiders at all. They just gotta make a living