Unfortunately, there's not a single entry for bitmap in the whole MiX \res folder.Not drawable, bitmaps. Bitmaps have to be png or jpeg format I think. If you change bitmap to xml or whatever it should work. It is looking for a physical image not a coded image.
From what I could find though, with vector drawables, Android has to convert/cache them as an image and when a drawable is used for 2 different sized images (like in this case: the files thumbnails and the album art cover), it is preferable to use 2 separate drawables - which is not the case in MiX.
In fact, the vector drawable works fine for the song files thumbnails (see one of my previous pic), so I don't think that adding an xml file pointing to the vector drawable will make a difference here. I usually do that for the opposite case: when I want to replace a vector drawable for a png.
Anyhow, thanks and no big deal; the last png I used is good enough for now.