Hi, I'm from Vietnam so let me answer your questions
Depends on how much you shoot really, but 40 rolls is about enough. You can buy these film in Saigon (PM me I'll give you the exact address and some notes):
Velvia 100, 100F, Provia 100F, Reala, Elitechrome, Superia 100, 400, 800, 1600.
BW: Tmax 100, 400, Acros, Neopan 400
Available in all size.
I'm not 100% sure but I think in Hanoi the choice would be a bit more limited: Kodak 160s, Superias 100, 400, Provia 100/F. For BW you'll find some Shanghai and Lucky too. I'll check with friends over there and let you know.
C41 & E6 is common, but there are only 1-2 reliable labs in Saigon & Hanoi that can do 35mm & 120mm, dont even risk your film in Nha Trang and Ha Long. BW processing is non-existent, you normally have to wait 2 weeks and the quality is hit-and-miss, so save it to process back home or use XP2/BW400CN.
This time of the year in Saigon there will be slight overcast with sunny interval, so for outdoor ISO100 is more than adequate. Hanoi will be a bit darker, so go with ISO400.
I can't comment on whether to bring more colour, it really depends on your taste. Saigon & Nha Trang will have sunny days, thus make for vibrant colours. Hanoi & Ha Long will be more muted due to overcast.
For camera store, new digital stuff I can point out a few place, just dont think about buying manual or vintage cameras, most of them are ridiculously expensive and not very well kept.
With regard to gear, your choice is perfect. I would recommend the 16-35 as it's faster and you can get close, people are quite cool to be photographed as long as you're obviously a foreigner. Hassy with 3 lens might be heavy to lug around, if it's sunny you're get toasted fast, so have a look at the weather forecast.