"What I read is that vehicle searches require probable cause. Reasonable suspicion isn't enough."
I would suppose that with probable cause they don't have to ask -
If they ask politely "Can I look in your car ? " and you say no, that might be suspicious...
According to this site, which provides definitions of probable cause and reasonable suspicion, "Refusing a search does not create reasonable suspicion, although acting nervous and answering questions inconsistently can."
http://www.flexyourrights.org/facts/probable-cause-definition.html
Once a cop has probable cause, they don't have to ask your consent to search your vehicle.
If they have nothing at all, not even a reasonable suspicion, they may still (of course) ask, but refusal is not cause for reasonable suspicion.
I guess the long and the short of this is that in Virginia, a cop has to have a more solid reason to search a vehicle (without consent) than they do to make an arrest of a felonious illegal under 19.2-81.6.