After buying Neymar for 220millions any transfers seem realistic now.
300 million of course is a huge amount, but Vinicius almost got the golden ball and was one of the most important players at Real Madrid, so he is a stellar and productive player.
For Real it will be just a gift of fate and I think he will gladly let Vinicius go. The only problem may be Vinicius' desire to play in Europe, because now he is 25 years old and this is the best and most productive period for the player and Vinicius may want to win more trophies in Europe, but I think Alonso is not very satisfied with his game now and may just put him on the bench.
Vinicius Jr. is fantastic and effective player and his near Ballon d'Or win and importance to Real Madrid greatly increase his market value. Real Madrid would likely see $300 million offer as huge bonus and might be happy to let him go especially if they have other players in mind or can use money well. However biggest issue is probably Vinicius Jr. own wish to stay in Europe and keep winning big trophies while he is at his best at 25. Leaving top European competition now would be massive choice as it could mean giving up more Champions League titles or chances at Ballon d'Or even for huge salary.