I don't think Vinicius wants to be recruited by Al Ahli at this time even though the offer is very big. The reason is, currently Vinicius is still relatively young. So you can be sure that for now he will continue to make achievements with Real Madrid. The reason is, playing at Real Madrid has been Vinicius' dream for a long time, so you can be sure he will not destroy his dream just for the sake of money. Apart from that, Vinicius' contract at Real Madrid is still quite long, namely until 2027 and if for example Vinicius' performance is good, it is possible that his contract with Real Madrid will be extended again. So, in my opinion, Al Ahli's hope of being able to bring in Vinicius Jr will not come true.
Vinicius is at Real Madrid, won big titles with them, and currently are their main star player with very good salary. He has little reasons to leave Real Madrid and move to Saudi Professional League. He is very young and it is not good time to risk his career by moving to a lower level league like this.
Maybe in future, like next 4 or 5 years, Vinicius gets older and achieves enough success he wants in Europe, he will take a super good offer from a rich club in Saudi Professional League, to collect massive money with several seasons in Saudi Arabia. But if he feels enough money in Saudi Arabia, he will find chance to return to Europe, like Neymar is trying.
But if we talk about Osimhen, I think Osimhen could be recruited by a Saudi Arabian League team like Al Ahli. The reason is, currently Osimhen's career is not going well at Napoli. Apart from that, many teams that were reportedly interested in Osimhen seem to have withdrawn because the price set by Napoli for Osimhen is quite expensive.
Osimhen has bigger chance to join a club in Saudi Professional League because he is at lower position than Vinicius Junior and if he joins a big club in Europe, his position will be uncertain too. No guarantee that he will win Champions League with any big club in Europe that is not Real Madrid. Osimhen can look at Sadio Mane and consider it as a convincing reason for joining the league.
A higher chance is when Osimhen is at similar age of Mane, he will move to Saudi Arabia, after getting enough challenge and experience in Europe. Money will be more important at that stage of his career.