What's the point of vector control for ac machines?
Your question seems to come down to this: Why would we want to control the rotor flux?

Leaving it as normal (at maximum flux and running with it orthogonal) would give maximum efficiency, but maximum flux also gives you maximum back-EMF. Without vector control, your speed will be limited to where your BEMF approaches your rail voltage.

Vector control allows you to do "field weakening". By reducing the strength of the flux, you also reduce the BEMF and can then run the motor faster than without vector control.

There are other advantages as well, but I don't understand them all.