Vlad is certainly good enough to get called up now. But he's projected to be a 108 wRC+ bat. How much does that actually improve the team? And then whatever improvement that is, is it worth losing a year in his prime? If he's the ~7 WAR player we all think he is, that extra year is worth a shitload, north of $50 million.
There's a reason the Cubs dicked around with Bryant, the Braves with Acuna, etc.