Currently ive noticed a lot of vehicles that are on the social club web site but are not found anywhere in the game outside of missions, here is a list of them:
HVY Cutter(This is a drill for those who dont know)
Nagasaki Dingy(with out rear bar and seats)
Police Prison bus(might spawn around the prison?)
Police Maverick(I only see Police Froggers)
Declasse Rancher XL(Snow)
I do know that the snow versions of vehicles are exclusive to Canada along with the Chain tires.
Edited because everyone says so.