Strikers quite often need to have a run of games to get into their stride, Vipotnik's never had that, he's in, he's out. Rarely gets more than one game on the bounce and usually substituted on or off.
Not saying he'd blossom with a run of games, just he's not had that. Strikers are usually confidence players too, so chopping and changing doesn't do his confidence much good either.
It could be he moves on and the next team plays him more and to his strengths, score goals and sold for 10 million+. Its the way it goes with us.
(yes he could be useless lol)