I switched to a fishpond chest/back pack a couple of years back.
The chest pack can be removed for when I'm fishing close to the vehicle. By attaching the backpack, I can carry some extra supplies (like a first aid kit, and extra water) when heading further away.
The problem I've had with a vest was I tended to overload. If it had a spare pocket there had to be something for that pocket. When overloaded it was cumbersome.
With a pack, I've gotten much more streamlined and focused on the things I really need. Everything has its place so I don't spend needless time searching for accessories. It has also slimmed down my fly selection. I find myself carrying a couple boxes of tried and true flies and leaving a lot of others back home.