Your only realistic hope of comfort in 100 degree weather is a mesh vest. Plenty of pockets here inside and out in a garment that screams worker guy, not gun. Obviously go brown for that low key appearance, although looking like a construction hand is a valid subtrefuge in an urban environment.
http://www.forestry-suppliers.com/iCat/ ... asp?page=9
http://www.benmeadows.com/search/vests
The famed Filson Cruiser vest above is great for fall/winter weather, available in tan:
http://www.benmeadows.com/search/vests/8984/