I've got a 5 year old Vios due for an annual service

What should be the approximate cost, assuming no extra work is needed.

Last year I took it to a Toyota dealership who quoted 10,000 but charged 8,500Bt

Rob, I think it depends more on the mileage interval than the year. Maybe best to check your Owner's Handbook for full details of what's required at your current mileage. I would then take the car to have the work done by a local mechanic....definately don't waste your money paying Toyota for their "service"!