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.
manchester united is also undergoing a major overhaul and is facing financial difficulties, so bringing in vinicius could worsen the team's financial situation. therefore, a transfer to manchester united is currently out of the question. the most likely option is a transfer to a saudi arabian club. besides being willing to pay vinicius's transfer clause, he could also gain the wealth and financial security of the saudi arabian club. however, the saudi arabian league is not as competitive as european leagues, so his talent could be wasted once he arrives there.
so, staying at real madrid is still the best option for vinicius. besides being able to play for a team as good as real madrid, he can also further develop his career.