Maybe this is related to their thinking of selling Rashford? However he is playing as LW the most this season. Therefore I don't know why they plan to sign a 3rd striker for the team. 

If they are thinking of parting ways with Zirkzee or Hojlund then I would understand. But I didn't see any rumour about these players to leave either.

I believe Vlahovic is more talented than both of those two though.    But if I were in their shoes, I would sign a LW instead of parting ways with Rashford. A really reliable one I mean.