If you buy an actual #9 you need to feed them chances. They're never going to pick the ball up 40 yards from goal, drive past 3 players and tuck it in. The plus side is, if they're good enough they should be lethal in the box.
Obviously Vlahovic needs to improve his overall game but beyond the basics it's not gonna happen.
What most teams do is end up padding their squad with 'forwards' that are way better at everything else except they miss 7 chances a game & there's no movement in the box.
Last seasons Chelsea & Bayern are great examples. None of those modern 'forwards' are ever in the right place at the right time, they all want to be the guy who makes the play.