A top-quality fly fishing vest is a massive benefit, whether you're fishing for an hour or all day. But if you fail to invest the correct amount of time getting your vest set up in the most logical way for the method you fish, you'll ultimately be spending more time searching for stuff than casting.
Choose a vest that has wide shoulder straps and neck support this helps spread the weight. I prefer a mesh back as it makes the vest lighter and also provides airflow to keep you cool.
A fly fishing vest is not a fashion statement it should be functional. I suggest you buy at least a size larger than normal also allow extra room so you can fit a good thick fleece under it in the colder months.
Other types of vest packs that are detachable from the pack are also worth thinking about. It's a personal choice.