How sideways can Vinni and United be discussed? It's like discussing Vinicius transfer to West Ham or Everton. Having the option to go to the Saudis, he will never go to a clear decrease in the team class (change of Real Madrid to United) without a multiple of income. Only Saudis can provide this opportunity. I do not need to think that he wants to go to Saudi Arabia, with a free agent, but after he works out his contract in real life, since he has the opportunity to win the Champions League and also claim a Balloon D'or.
He is playing for a biggest and most successful club in Europe so it is impossible for a transfer from Real Madrid to Manchester United. He has good chances of winning more titles with Real Madrid while with Manchester United he will face with mountain of challenges and will almost not be able to win any title.
If he wants money from lucrative salary, his destination would be Saudi Arabia with many rich clubs there are ready to give him very mazing offers that can help him richer in a few seasons.