It doesn't matter if the seats are leather or vinyl, you can assist water-proofness, by using small amounts of Kiwi black shoe polish and/or mink oil and scrubbing it into all the seat seams with an old soft toothbrush (same as you should do with leather boots). If you want to melt it in better, use a hairdryer/blower (don't burn the vinyl). Wipe off excess.
I have two sets of Ultimate vinyl seats and have been the the rain a hundred times (and get them wet washing the bike), and my seats have never taken on any water. Never bought the rain covers. But what I do not do is leave my bike just sitting out in the rain for hours or overnight. When traveling, I use a small light half cover (packs to softball size)*. It's not fully waterproof (very resistant), but it keeps the seats and electricals from being exposed to hours of direct drenching rainfall, most just rolls off the sides. And keeps tourists from putting their kids on my bike for pictures.
*No hooks or metal, easy on and off, leave it in the shower to dry. Full size covers take up way too much storage space when traveling.
http://www.amazon.com/Dowco-26014-00-Guardian-Traveler-Motorcycle/dp/B000GROHP6/ref=sr_1_16?ie=UTF8&qid=1439671912&sr=8-16&keywords=motorcycle+cover+dowco 
