To those still wondering about this, there appears to be either a bug or a not-so-user-friendly feature of the texture sampler in some or all versions of the engine.
This holds true for both the regular Texture Sampler and for the Texture Sampler Parameter nodes.
As a sidenote: When you have a TextureSampleParameter2D in the Material Graph, it is displayed being called just Param2D, which can itself be a bit confusing. You can see the full name of the node type by hovering over the node and pressing Ctrl+Alt.
When you click on either of these types of nodes, and try to browse through the details panel, you will find a dropdown box called Sampler Type. This box is a bit misleading, because it doesn’t actually show all the available sampler types for these nodes.
Instead of using this dropdown box, you need to go back to the Material Graph, and click the little “v” shaped arrow at the bottom of the node to expand it unless it is already expanded.
Now you will see a dropdown box there also named Sampler Type.
This is where you can switch between using the regular texture sampler types and the virtual texture sampler types.
In the details panel, only one or the other type will appear depending on what is set within the aforementioned hidden “v” area Sampler Type dropdown box of the Sampler nodes.
I hope this helps someone!
