First you need to create the Level of Discount you want to use. Once created it be used again for other offers

Login to VirtueMart

On the Administration Panel go to Products > Add Product Discount

Enter the value for your discount and click either the Percentage or Total button.

The start and end date options are optional

Navigate to the product you want to add the discount to
Administration Panel > List Products > Click Product

On the Product Information tab select the discount you have just created from the Discount Type dropdown

Save