It does not tell you to use more or less throttle, that would get too annoying too fast
The way it will tell you that you are in lean burn is by the injector pulse width. It will change when entering lean burn, and you will notice a loss in power when your car enters lean burn. I've never driven a VX, so one of the members who have will have to tell you how yo get in lean burn.
And yes, a vacuum gauge will not be needed. If you find a BSFC map for the D15Z1 engine, that will tell you the load needed at certain RPMs to have the most efficient acceleration and cruise.