It matters (unless your minSdkVersion is Q or higher) because theOutline API explicitly only accepts convex paths — see the documentation. When running on P or lower, you can only pass a Path for which the isConvex property is true; if it’s false you’ll get an error (or no outline -> no shadow; I don’t recall). Starting from Q, you can also pass a concave path, but it’s unrealistic as of today to assume you can always do that given the minuscule fraction of users on Q.

In any case, you need to construct a Path that matches the vector drawable’s outline you want the shadow for, and pass it to the ViewOutlineProvider for your view. Some pseudo-code:

private fun setOutlineForDrawable() {
  val myImageView: ImageView = // ...
  val myVectorDrawable = myImageView.drawable
  val outlinePath: Path = // ... You need to figure how to create
                          // this by yourself, I can't help with it
  val outlineProvider = PathOutlineProvider(outlinePath)
  myImageView.setOutlineProvider(outlineProvider)
}
private class NarrowerOutlineProvider(
    private val path: Path
) : ViewOutlineProvider() {
  override fun getOutline(view: View, outline: Outline) {
    // Optional: transform path to adapt to view bounds
    outline.setConvexPath(path)
  }
}