In the context of B2B it's not at all odd, you're right. (In fact, that's the whole reason transfer pricing rules exist.)

In the current era, it is however uncommon for consumers to simultaneously own and (exclusively) be the customers of a company. (Historically, that is of course the entire principle of mutual funds.)

This is what co-ops are, and as far as I know they do not face special tax reporting or payments.

Depending on structure, co-ops often do face special tax reporting requirements.

Moreover, in this case the fact that Vanguard is a co-op is a redeeming factor. If it were a typical corporation charging itself below-market transfer prices, that would likely be illegal.