Yes, you can. If you purchase VIP when you have already purchased Premium then your VIP will take over the Premium. Once the VIP expires or you cancel it, you can use the remaining of your Premium.

 
For example: 
A User purchases premium for 30 days. On day 5, the user upgrades to VIP for 30 days.
The remaining premium status becomes extended by the VIP duration, meaning the user has 25 days of premium if/when the VIP expires