I have to weigh in on a small back-pack rather than a waist-pack. Maybe it was because I was in the army for 9 years, but I find a backpack to be totally unnoticable. I have worn a waist pack before but it seems to throw my center of balance off (something to consider if your birding day includes carefully stepping across stones/boulders to cross a river).
Don't worry about birding "fashion". Loose, comfortable, drab clothing is all that is required.
A hat with a brim is a vital necessity - you need something that will block the sun while you look up at birds. I find a simple baseball cap to work perfectly well although many here swear by Tilly hats.
A vest is fine if you actually need a vest. The days of film photography are mostly behind us though. What would go in the myriad of pockets the vest offers?
I used to carry a field guide (actually two - my Peterson's in my right rear pocket and my Kaufman's in my left rear pocket), but haven't carried one for the past year or so.
Plus, vests are hot and confining.
Loose, drab clothing is the way to go. Make sure you have a brimmed hat. I recommend a small backpack for outings that involve any significant amount of hiking so you can carry lots of water, power bars, plastic ponchos that can be used for sitting on, a first aid kit, bug spray, more water, etc.