John, I have used both the backpack and vest and to be honest I prefer a lanyard. I like to go fishing with as little weight as possible and the lanyard is very simple to make and you can put as many swivels on it as you need for your tools and tippet. As for a "hydration system" a simple and cheap idea is to go and buy a fanny pack with a sports bottle included, usually under $10, and you would have that compartment to put an extra fly box or two in. If you decide to make a lanyard and try it out let me know and I can send you some surgical tubing for the part that lies against your neck for comfort. If you need instructions on making one let me know and I can send them to you.
Bart
