Use custom imported asset as texture property

[red tendon]

You don't have to "allow the file to be dragged into texture object slots".
Instead, you'll need to tell Unity how to import .vtf files as Texture objects.
To my knowledge, the best way to go about this would be using a Scripted Importer.

In that case, you might need to show your code and specify what exactly you are doing to import a file, and at which point it stops working as expected.

Okay so... why do you have this importer in the first place?
It literally doesn't do anything.
It doesn't even fulfil the minimum requirements explained by the documentation:

tried to add what?
what accepts what as a simple object?