That only applies if it is, in fact, legal. We have specific laws in place to prevent corporate tax evasion through unrealistic transfer pricing, and it's unclear if Vanguard falls afoul of those rules.
Yes, in general that's the goal of transfer pricing. However, the way the rules are written it's not clear if they also apply to this case, particularly because the IRS does not recognize intent as a factor in transfer pricing rules.
To be clear, I do think this should be legal and kosher. But I sort of see how the argument could be made.