Little late here but I hunt with a Chessie and he’s got his vest on anytime the air or water temp is much less than 50. A lot of folks will say that dogs (especially chessies) are made for the cold and don’t need them, but I’d rather have a dog that is comfortable and enjoys jumping in the water than one that’s cold and miserable the whole time.
Side note: My dog lives inside. I suppose if you have a dog that lives outside in the cold he builds up a thicker coat that will keep him warmer. But I don’t know why anyone wouldn’t want their best friend in the house with them.
For me having a Springer it is a little different. If I know my dog is going to be retrieving in cold water, if it's close to freezing outside, or sitting for periods of time in cold temps...the vest goes on.
I had a vet tell me 30 years ago that putting a vest on your bird dog will add a year to their life. As the less their body is fighting to stay warm, the better off they will be not only during the hunt, but long term. Made sense, and I've been using the vests in inclement weather since.