a medium sized backpack should be sufficient for a 3 week trip here..
just bring a few light clothings.. buy the rest here.. it's really cheap.. t-shirts can go for around usd1.. there are amazing tailors in hoi an that can sew up anything as long as you have the pic or can describe to them and it's all done in a matter of hours..
generally toiletries and medical supplies are easy to get in the big cities.. just have the basic with you.. medication for bad tummy may be a good idea if you're planning to be adventurous with food.. another idea, they have almost any creature soaked in wine for all sorts of medicinal puposes.. try it if u dare?

having spare film and battery at hand will save you the time of hunting for one when you need it.. it's not difficult to get it in the cities but it might just decide to give way up in the mountains or out in the waters..

if you're planning to stay over nite on the boat in halong bay, bring lots n lots of mosquito repellant!!
bring a hat or buy one.. the vietnamese hat is cool and perfect for shades from the blazing sun..