use f5.5 and above. when you saying group tours doesnt me you always take picture in a group.
what you need from this trip is to show people you have been in this place by shooting an object with the place landmark or scenery. in such case you may need f-stop to have both object and background in focus nicely.
it will be very rare to use zoom lens, therefore to me up to 55mm lens is more than enough.
i went on a tour of NZ a couple of years ago and brought 3 lenses. but a 28mm f2.8 stayed on my camera about 90% of the time. if i shot people, i just moved in closer, and it covered most of the scenery and landscape shots. the other 2 lenses were dead weight. hahaha
Optical quality of the 18-200 is only slightly better than the 18-135. The range is also not very much longer. Although VR might help a little, I suggest you work on your photographic skills, eg composition, exposure, etc.. instead because I don't think the 18-135 is such a bad lens.
Just what exactly are you not happy with the quality of the images from the 18-135mm lens? You can make good quality shots with any camera, so long as you know their limits and strength.
I like Ho Chih Minh, and its pretty alright. Just don't be flashy. There are loads of tourist there, and the city itself has employed tourist security all through the main sections to help tourists cross the street and prevent theft. I went there in Apr 06 and mostly use my D50 with 18-55mm lenses. As you can see, I can't keep changing lenses. I can't say I have get fantastic photos, but they are on my website for your reference.
18-135 has enough reach for travel photography. If you want to rent a len for the trip, you have got to get something you are familiar with, else you will be still figuring out the limits of the lens on the trip. Of course, nowadays, I mostly travel with the 18-200mm VR...