Cambodia is a shit hole. Some parts of Phnom Penh are dreadful. Sihanoukville is nice though for a week or so then gets a bit boring. Haven't done Siam reap because I couldn't be arsed with a monstrous bus journey, they bore the fuck out of me and the Cambodian buses are shit. If you like go go bars, then don't bother in Cambodia because they all reek of BO.
Vietnam is a lovely country. Ho Chi Minh City is quite developed for a South East Asian city, and the banh Minh sandwiches are banging. Hanoi is also a nice city, bit mental with all the motorbikes! Halong Bay is alright, I was there a few months back but the weather was shit for the few days I was there. Looks like paradise when you look at the pictures, but when I went it was misty and it resembled cleethorpes in November. Beer is so cheap is Vietnam. I was paying 20p a beer in Hanoi then £8 a beer in Hong Kong a day later! I'd go back to Vietnam again to do the coast line on a motorbike, I didn't have time to do that.