Hello,
VIP Numbers carries value when owner can transfer it new car
But offlate - Old registration numbers are not what easily been transferred to new cars and in some of the states the norms are with a clause of some 3 yrs in which Old number should be retained in before proceeding for transfer to new car
So - unless you find a buyer who can do same, the value of 15 year Number despite of VIP number remains as good as 15 year old car with standard registration number
With respect to retaining Old Series and transfer to New Vehicle by Seller
Refer in below articles
http://timesofindia.indiatimes.com/city/mumbai/Now-you-can-use-old-number-on-new-car/articleshow/7479273.cms
http://www.ndtv.com/article/india/soon-keep-your-old-number-plate-for-your-new-car-18220
http://timesofindia.indiatimes.com/city/nagpur/Now-retain-your-old-lucky-number-for-new-vehicle/articleshow/10241165.cms?referral=PM
Hope should help !!