I can't say what specific temperature is OK, but I will say I wouldn't force it on her during winter. I have a pup just slightly older than yours, and I don't plan to let him swim until it's quite a bit warmer and he has a thicker coat. If we have a mild day and there's a shallow puddle for him to splash around in, I might let him and see how it goes, but it won't be in a training session.
Ours was born in December so didn't get exposed to water until ice had been out and water warmed up a little in May. Now you can't keep her out of the water, so waiting clearly wasnt detrimental. Don't let the first experience be a bad one. It will already be scary, so it helps if it isn't scary and painful.
Make it fun for your pup, fun-fun-fun. Play with him in the tub maybe, if you want to go now. I always get mine crazy retrieving a certain "toy" and then toss into shallow water, say ankle deep, then a little more and a little more. I start when it is warm and I am sure it will be a good experience. You can start with a toy in the tub and let him splash around. Makes me wish I had a puppy now that I think about it.
I've used vests on my labs and hunted with them into the single digits. They were acclimated. Few things are as tough as a good lab.
I think it depends on the dog, and whether they are acclimated to conditions. We have waterfowl hunted with ours into single digits with a vest, but you have to let them move around more to keep the blood flowing and give them a dry protected place to sit. She's a fairly big girl though (75 lbs) and loves the cold. She certainly doesn't handle the heat as well as some smaller labs we know, and they don't seem to handle the really cold as well as ours. But put some ice and snow on the ground and she goes nuts! We take her upland hunting in the -20 stuff with just her skidplate and she's happy as a clam.
One thing we've noticed though - we only put a vest on her if she's going to be in the water and take it off as soon as we get to the truck. She's always dry and feels warm underneath to me when it comes off, but she acts like she's cold if we leave it on and seems to warm up fast once it's off.