It's admittedly speculation, but I always assumed that Vaarsuvius had the same barred schools as Aarindarius. Which is to say, Aarindarius taught V, including passing on their own limitations. V's parents just apprenticed him to "that Wizard friend of ours," rather than scouring Elven lands for the most optimized caster.

Yeah, V does say that they chose Conjuration as a barred school, but it still seems plausible that they did so because they didn't think working with multiple instructors was a good strategy. V definitely seems to be of the mindset that it's better to be excellent at one thing than very good at several things.

Now, maybe Aarindarius should have suggested V study with someone else, but they didn't. Maybe they suffered from the kind of myopia that seems to widely afflict Wizards in this setting: they all have a tendency to assume that their field is The Best and look down on people who made "inferior" class and build decisions. And while we see this more in terms of Wizards looking down on non-Wizards, I think a milder version applies between specialties as well.

TL;DR: the simplest explanation would be that V took the exact same specialization and barred schools as his mentor without really thinking about it.