Sounds like an amazing trip. I visited Hanoi, Ha Long, Siem Reap and Bangkok last year (DSLR skills not up to much at the time, but I feel as if I've improved a bit and can do the places more justice when I next visit).
It sounds as if you've got all the bases covered. A couple of things I would add to your bag though:
lightweight tripod (if you're travelling to the temples at Angkor that will be useful - sunrise is not to be missed)
cleaning cloths (lots of them, as the humidity in the early mornings could mean you'll miss a shot otherwise)
small torch (again, if you're up early around the temples, you'll need one to watch your step)
insect repellant (or you could be eaten alive)
Hanoi is street photography heaven. I found that I never went above 105mm - you could get away with light walkabouts with the 15-85, but I'd carry that 70-200 with you for more candid stereotypical shots (scooters, market stall owners, women walking around with conical hats etc)
There are a few places where you'll be inside (like the famous prison) where a 1.4 could be useful as well.
Halong Bay - awesome if you get good weather. A lot of people on another forum suffered the same fate as me (i had slight haze, but others have had thick fog). You'll have everything covered with what's in your bag but might like a tripod again as you haven't got a fast wide angle for the caves if you visit.
Cambodia - pips the lot, I think! Awesome people, beautiful country, and those temples have to be seen to be believed! I'm planning an 8 day trip in 2011 to try to do the place justice and really take my time. I took a 70-300mm with me last time and never used it. I was using an 18-105mm kit lens which covered just about everything, although I wish I could have gone wider with some of the temples (14-24 Nikkor will definitely be with me next time)
A very useful guide for all things Cambodia as a traveller is:
That link takes you to Siem Reap, but there are links to everywhere else in the country as well.
Have a fantastic time but please don't make me too jealous!