I think you will be fine with your 18-50 tamron. the 50 mm end will work fine for people shots, its a normal length, so no distortions there, kwim? :)
and i'd practice a little with your on camera flash and high isos. if you have your aperture pretty wide open and your flash on slow sync your pics won't be half bad. but i'd run thru a few test shots at home first, just to see what the results are. Its my opinion that some decent pics with on-cam flash are better than none at all! :) I have had too many occasions when my 50 1.4 is not wide enough and stepping back doesn't work so well. so i am finding the 18-50 range to work very well. I really think it will suit you fine for light travel. :)
hth!
~~Tammi~~
d300, MB-D10, sb800, sb400, 18-200VR, 18-50 f/2.8, 28-75 f/2.8, 90 f/2.8 macro, 50 f/1.4, Pocket wizards, sekonic L-358, and an aresnal of reflectors and other light modifiers....and maybe more. ;)
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