Is the ability to change the [Vanguard's nose] mount coming in 3.7?

Yes. Technically, you can already change them. You just don't have anything to change them with. All four variants come with different nose mounts by default, and they're all available in the shop. So if you're not happy with what comes with your ship by default, then you can go to the kiosks and swap them out for other things.

Are there geo changes with that?

Yes. They've all got unique geo. They've all got different audio and VFX.

What does that do to the livery of the ship?

The guns themselves are generic, neutrally colored so as not to interfere with any of the paint jobs on the parent.