There is no big download involved for the vector maps update. It is for the center console (Google Maps) only, and is switched on server side. This has nothing to do with nav - neither routing, nor IC display. What you get out of it is that the zoom-in, zoom-out, rotate, (and lateral movements?) on the center console will appear smoother. Transitions will be smoother because what is displayed is dynamic vectors of tiny images/icons overlaid on each other rather than tiles of static images used before. I don't know if I expressed it accurately, but hope you got the idea.
If you ask me, I am still not convinced that this upgrade is connected with the 2018.12 firmware update or the nav update. But some people have reported to have received it at around the same time. I haven't seen enough people reporting it to be convinced that it's not just a coincidence. Most people are also confused and don't really understand what vector maps update is, so some may be incorrectly reporting it. Another factor is that it is not very easy for everyone to tell if they have got the upgrade. Many may mistake faster zooms to be vector maps upgrade. I have not seen any reports of explicit indication of vector maps upgrade, so you just have to figure it out by zooming in and out. Also, there is a chance only people with newer MCUs will get it.
I have a classic pre-AP MS. Old MCU. Haven't got vector maps yet. Got 2018.12 and the new nav.