Marios Forsos , May 20, 2009; 05:50 p.m.
I travelled through Vietnam last year (albeit not during the monsoon season) and it must be said that, depending on where you stay, humidity can indeed be a problem. Let me explain: if you stay in air-conditioned rooms, coming out from them in the morning will cause your lenses to fog up pretty quickly and it can take upwards of an hour for the fogging to go away (plus the unknown effects all the dissipating condensation may have on the lenses themselves). The bigger the temperature difference between the outside and your room, the more severe the problem.
Having said that, if you simply adopt the simple: stick the camera in a sealed plastic bag before you go into the room (and the vice verse of course) and wait for temperatures to equalise, should solve most of your problems. However, there is another piece of advice for you:
Even though I've never gone to Vietnam during the monsoon season, I have travelled through Thailand, Malaysia and Laos during the summer months and let me tell you, a raincoat is NOT enough. Not by a long shot, not if you want to shoot when it's raining (or even carry your camera relatively accessible and not buried in some case and sealed against the rains - which are heavy and prolonged!) You would be wise to invest in a rain-sleeve for your camera because, let me tell you, the sealing in the Canon leaves a LOT to be desired. I was lucky enough to have my trusty Nikons with me which handled the rain admirably well. I read quite a few tests of people taking the 5D Mk II into wet environments and having the camera go belly-up on them in pretty short order. A rain-sleeve costs around 10 bucks and will keep you shooting even in torrential rains