I haven't tried the WD40 specialist water resistant silicone but I can recommend something that works great on rubber and vinyl (and plastic).
It's called, AEROSPACE PROTECTANT® 303.
The label says "SPF40 For Your Stuff" and it works great.
With regular use it provides 100% protection from UV.
Ultraviolet light is one of the things that can cause rubber, vinyl and plastics to crack and fail.
I'm also sure it protects from ozone which is another thing that can break down rubber and vinyl.
I use it on the vinyl top on my Miata and it works great.
It is often found at boat shops and it is also available on line from many places.
Just be sure the name of the stuff you buy is spelled exactly the way I wrote it at the start of this post.
I have heard there are some copycats around with shoddy stuff for sale.