Trip to Vietnam, which lens should I buy and should I bring my tripod

Hey Everyone,

First I'd like to say hello to everyone, always been a fan of DPreview but have never asked questions before.

Im going to Vietnam for 3 weeks and am unsure of what kit to bring.

I have a 500D,
Canon 17-85,
Canon 50 1.8,
Sigma 10-20,
old canon 75-300 (which I plan to leave behind)
Manfrotto 55XproB tripod

I have been considering buying the new Sigma 17-50 2.8 OS (newly released) to replace my 17-85. I am hoping the additional f-stops+stabilization will help me capture some nice portrait shots. Can anyone think of a better lens that would suit me under $1200?

I was also going to get the Sigma 70-200 2.8 OS but since it has come out with a street price of $1700, its too expensive to get before the trip. Does anyone think I can get away with bringing no telephoto options?

I would like to travel light, however I spent two months travelling across America by bus last year and really regretted not bringing my tripod on that trip. I discovered that while I am in Vietnam there will be a coloured lantern festival one of the nights which I would love to capture. Should I 'lug' my tripod around Vietnam even though I might not use it more than once?

Thanks for your time
Nick