Davywalnuts said:
think it will be a week in each for me. Will find out more sunday.. terribly underprepared!
Have you got your visa's sorted? You'll need them for Nam and Cambodia....
A week isn't long to experience Vietnam....we were there 3 weeks and had a couple of days at the main places.
If I only had 7 days in Vietnam I'd go from BKK to Hanoi (stay at Hanoi Packpackers...and say hi to Ngoc behind the bar from tom and will!)...experience the HCM mausoleum....street food...errm...possibly do Ha Long Bay. We did a 2 nighter, but if you're not there long than maybe just one day? We didn't enjoy our trip around ha long bay as much as other stuff.
Then, down to Hoi an, get yourself some made to measure shoes - test your bartering and get a good banh xeo...avoid the restaurants by the river (nearly all were crap). There is a good little touristy street food market thing down by the river which was really good though!
Nha Trang is good if you want a day on the beach...Vin Pearl land is amazing fun!!!
HCMC...didn't really like the place. Went to the tunnels, pretty boring but only cost $6 or so. Couple of museums are worth visiting.
Then from HCMC you can get the bus over to Cambodia...and I wish you good luck
You definitely need to eat street food. We didn't have any food problems...just eat where it looks busy! We had these amazing deep fried pasties!!
Take a "moto bike" taxi...good way to get around hanoi...just wait until you start going up the inside of right turning lorries - incredibly fun!!

(seriously, take them around!)
If you stay at hanoi backpackers (book in advance...email them or something - they were busy)...get them to pick you up from the airport. It wasn't that expensive and it saved us getting stranded there/wasting time fussing over the cost of getting into the city.