The service plan will cover your servicing for two yrs. Check with the dealer for the cost of a 1st and second service and then compare them, dealers price these differently. They will generally make 30-40% profit on all services on offer.
With regards to the warranty issue, no you do not have to have a service by VW whilst under warranty a thing called block exemption stopped this many years ago to prevent dealers having a monopoly over customers. Any garage can service your car providing that they use genuine VW parts and they follow the VW recommended service schedule. Ensure that you keep a record of these along with the service invoice as proof it has been done.
I hope this helps.
If you wish more information, please continue to ask