You've made a wise decision to travel to Southeast Asia. My partner and I were there last April. We were in Cambodia only to see temples at Angkor, specifically Angkor Wat. This is a must see. We would like to go back to see more of Cambodia. As for Vietnam, you should be able to travel the whole length of the country from Ho Chi Minh City (Saigon) to Hanoi. But take a few days off to stay at a beach resort on the South China Sea called Ana Mandara. It is located in Nha Trang. Five star luxury at bargain prices.
http://www.sixsenses.com/evason-anamandara/index.php Splurge for a seaview or deluxe room.
See the town of Hoi An, a UNESCO World heritage Site. Well worth the trouble to get there. Don't go out of your way to see China Beach unless you happen to be in the area or have a Vietnam War interest. Check out Hue, another World Heritage Site.
If you can get up to the mountain area in the North near China, go to Sapa. It's a beautiful town up in the mountains. There is a "luxury" train that leaves from Hanoi to Sapa. The whole thing can be booked through Victoria Hotels there. Stay at their hotel in Sapa. It's really nice. There are small villages you can travel to with a guide to meet the locals.
That's all that comes to mind at the moment. Get a good guide book and you are all set. Vietnam Airlines within the country was just fine to get around- modern planes, no one carrying chickens on board- but they are a little loose with the schedules. Leave a lot of extra time if you will be flying around.
We were not clued into the gay scene there. We went to a "mixed" bar in Hanoi, but it was just a little strange. We encountered no problems just being us.
Bangkok is...well...Bangkok. You'll have a blast. Make sure you see the all the tourist temples. They are really beautiful. Hire someone to take you out on a fast longtail boat through the canals. We stayed at the Oriental. Five star luxury at five star prices. However, there are more reasonably priced nice hotels that I'm sure other FT'ers could recommend. Have fun!!