Virgin Red will clearly state how many Virgin Points you'll earn with.
In most cases, this will be expressed as a number of Virgin Points earned per £1 spent with the retailer. If a retailer is offering 2 Virgin Points per £1 spent and your transaction total is £50 (excluding VAT & shipping), you can expect to receive 100 Virgin Points.
Another example, you may not earn Virgin Points per £1 spent, but rather a certain number of Virgin Points for a single purchase, or by registering for a service with that retailer. In this instance, you'll receive Virgin Points for that one action, but not subsequent actions or purchases on that retailer's site.