Your process can first have the user select a Campaign in a form of RegularUser. The RegularUser form can have a custom button name like "Search".
LoggedInRegularUser.SelectedCampaignName will then be available to use in the query.
By the way, if you are just trying to display the Campaign name above the query, there are other alternatives to the Banner section of the VP.
You could also just have a VP that has a form with RegularUser.SelectedCampaignName in a drop down. This attribute can be flagged to "Save form when selection changes". Hide the buttons on this form.
Place the "Selected Products" query in a Content Panel below the Campaign Form. This query will have RegularUser trigger an auto refesh BO.
So, the result will be that every time you select a new Campaign in the drop down, that Campaign's products will automatically show in the query below.