I'd be very wary of blending RUG in any amount in a newer car (IE, newer than 617). The alcohols in it can damage IPs I've heard (which is the reason those engines strictly warn you to not use any alcohol based fuel additives).
To be honest, blending like that is something you can get away with on many older IDIs but its tricky business. If you are going to do SVO on a newer car that you care about, take the time to do a proper two tank system.
France doesn't mix vegetable oil into their fuel, they mix biodiesel. Yes it starts its life as vegetable oil, but biodiesel is chemically changed and becomes a whole new product, the two things aren't interchangable.
Good luck, and keep your eyes on the SVO forum at
http://biodiesel.infopop.cc for blending advice.
peace,
sam