The VIP subscription doesn’t erase the old VIP you have. If you already have VIP status and you subscribe, the new VIP will add up on the old one. 

For example, if you have VIP for 3 months and you also subscribe for 1 month, you will receive 1 month VIP each month, but at the same time you will keep your 3 months and your profile will always show 4 months VIP after the new subscription arrives.