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.
I mean they are literally paying 200 million a year to Ronaldo already, he has been there for 2.5 seasons so far I believe, which means 500 million, on top of that they are going to pay 200 more for this season as well so that is 700 million. So for 3.5 seasons of Ronaldo, they already paid twice as much as Vinicious price, not really that shocking. Also, they do this for marketing and PR, which means if they can get Vini somehow, they could literally use that to say "we have the best player in the world playing at Saudi league".
Now, would I agree that Vini is the best? No, I do not believe that at all. But they can try and do that, it would make sense and that worths a lot more money for them. They have (so they claim) the best that ever played, and also the current best, all playing at the same league. Imagine the marketing.
I didn't know CR7 had such a huge salary, I thought he had a salary of around 100mil and if your info is correct then Ben Jacobs is right and the Saudis are already looking for 350mil for Vinicius. To be honest I'm not sure that Vinicius will be as marketable as Ronaldo in terms of marketing, because even despite his age Ronaldo was recognized as the best player in the world for many years and only Leonel Messi could compete with him.
So if I were the Saudis, I would not hurry to give such money for Vinicius, or it may turn out like with Neymar, who played 5 or 7 matches per season between injuries, besides the character of Vinicius is not the easiest.