1. You go to back office than -> Shop parameters -> Customers settings -> Groups (tab) . Here you create "VIP" group and add your clients here ( you need to select a client from "Invited" "client" groups and modify selected clients - choose those you like to see in vip)
If you like all VIPS has 25% discount for all products:
2. Go to backoffice -> Catalog-> Discounts. Here you add 25% discount, but dont forget to see and understand all tabs and params (play with it).
In you case, you need to Conditions (Tab) and select "Customer group selection" and then Actions->Apply discount and here you add your 25%.
If like to apply only for some products:
Go to your product you like to apply discount only for VIP:
Catalog->Products-> [select your product] -> Pricing (tab) ->Add specific price (button). And choose you params ( you choose for VIP group)