Do you have an REI or someplace you can try before you buy?
If not, I think Deuter offers a vented back that has an adjustable torso length.
ETA: I was wrong. Only adjustable vented back they have is a 42liter and it weighs over 3 lbs.
Also, I may have interpreted your meaning wrong when you say the hipbelt doesn't fit right. I assumed it was a torso length issue, but maybe it was just the design? If the later, then you probably want to look at other brands, as a smaller version of what you have will probably have the same issue.
Deuter does have a 38 liter vented back for 2lb. 11oz.