My problem is fairly simple. I have set this url in the Configuration>Page Delivery settings. The VEC tries to load this page, but our website auto-redirects it to the homepage because it doesn't have any order information stored in Session Storage (which is where it would be if a user had gone through the normal process of picking a product and trying to order it).
Because the page is redirected, it seems Target freaks out and the VEC becomes useless. It takes a long time for the VEC to "try" to load the page, then most of the links in the VEC menus seem to quit working.
So how can I set up an activity to perform some tests on our payment page?
"You can also click Browse, then navigate to a page that is available from the primary page, such as a shipping page or shopping cart, and test changes on that page. You can also access page elements that are available when you hover, such as flyout menus and mini-carts. When you are finished browsing to the page, click Compose to edit the experience. For example, you might want to change the design of a shopping cart drop-down or a carousel of images. This functionality is currently available for A/B tests, A/B tests with Analytics, and experience targeting."
Have you tried going to the site and putting something in your cart?
This approach worked for me on a Shopping Cart Test I needed to set up. The elements that were in the Test would only show up if there was something in the cart. So, I went to our site and put some stuff in my cart. Then, when I opened Target to work on the Test, the shopping cart that I created earlier populated in the VEC so I could manipulate the elements.