I have never dealt with virtual textures. Do these accept Texture2D or material variables? My plugin creates these.
Unfortunately the texture object in unreal only accepts uncompressed data. I can check if it works differently in newer UE versions. The last time I checked it, UE took the pixel format with the height and width of the image and calculated the size from it. If this did not match, the app was closed. And it only matched with uncompressed data.