First of all congrats on organising your trip!
Macka and I spent 1 month in Vietnam recently. All we took was the 18-70 kit lens on a D70s, a Canon P&S, plus Manfrotto tripod. Our lens choice was more dictated by lack of funds than anything else, but if we'd had the choice we'd probably have taken a Nikon 18-200 & either Sigma 10-20 or Tamron 12-24.
I'd go for the Nikon 18-200 over the others you mentioned purely for VR - the number of times the ISO went way up because of low light was huge, and this would be even more of a problem at longer focal lengths.
If you're really travelling light, I'd consider dropping the 12-24, it'd be bad to lose the wide end but the weight saving (as well as avoiding the inevitable dust bunnies) may be worth it. It really depends on your shooting style. If you take multiple lenses, remember that dust will be a problem, and you'll have to take some sort of cleaning kit as well - definitely a blower, and possibly a wet cleaning kit.
Storage is a real issue, particularly when you leave areas with lots of computers. An image tank is a great idea, but at some stage during your trip you're going to want to burn DVDs, mail one set home and keep another lot with you. It's a hassle, but better than losing the lot if the HD dies
Tripods & backpacking don't mix well - I know as I lugged all 3kg of solid metal several thousands kilometers. What sort of tripod do you have, a tabletop (ie 10cm high) or just a short one (~50cm)? We got some of our favourite shots with it, but then again, we were only going for a month, not a year. A beanbag (or better yet a rolled up jumper) gives you some options for long exposures, it's not ideal but neither is carrying a tripod easily
Finalmente, ¿hablas español? Learning a bit of the language will make it much easier to get around. Here's a useful phrase to start you off - No soy estadounidense (I'm not American) - the locals will like you more straight away
I've never been there, but apparently there are stunning beaches in the Pacific coast of Mejico south of the Yucatan, but without all the Cancun-style development. Make sure you eat some cucarachas while there - my Spanish teacher liked them fried

. She was Peruvian (and hence biased in this matter) but always told us that if we went to Latin America, Bolivia and Peru were the 'real' places to see, especially up in the mountains. Friends who've travelled in Cuba say to get out of Havana as much as possible - it's devastatingly poor, but the people are wonderful and the country fascinating.
Patrick