Building my girlfriend a bike
I'd love to get her a B18 but I KNOW she will eventually leave the bike out in the rain and the saddle will be ruined.
So I would like to find a saddle that has the classic and sophisticated styling of a Brooks but the weather resistance of a vinyl
Honestly, a couple of times getting wet, especially if it is not ridden while wet, is not going ruin it. My B-17 has gotten soaked about 3 times now and is fine and I've heard similar things from other Brooks users. Heck I even rode mine in the rain 1 of those times and it just ended up making it a lot more comfortable (I am a lightweight though).
I can't imagine you will find anything like a Brooks in vinyl and even if you did, vinyl may look a bit like leather on the surface but it doesn't breathe or wear in nearly the same way.
For that reason of possible rain, I chose to go a somewhat different route. I use Nirve or Electra saddles intended for cruisers. I find that they are just as comfortable as Brooks for me and I do not worry about rain.
I don't mean to doubt her too much I just can see a situation where she forgets to bring the bike inside or forgets to put a cover on and it sits in the rain all night. Just can't be good for leather. The rainy season is just now starting too and it can rain unexpectantly.
It isn't the water that hurts Brooks, it's the stretching that occurs when you ride it when it's wet. Let it dry without weight on it, and it will be fine--but that can take days.
I goop mine about twice a year with Proofhide and carry a heavyish plastic bag (thicker than a grocery store bag) to put over it in the rain, and it's 12 or 15 years old, It's been wet many times, but never ever ridden that way.