How do I take a sequence and some vocal tracks and mix it down 2 tracks - or a stereo track?

There is no one way to proceed. Your options will depend on which version of Cubase you are using and just how you've setup the Tracks. Are you using the Yamaha MOXF6/MOXF8 Editor VST? If so you can use the Cubase feature EXPORT AUDIO MIXDOWN.

By routing your Cubase MIDI Tracks through the Editor VST, you can setup to return audio to Cubase - this returned audio can be used to render Audio Tracks as you require. You have two stereo buses available. If you wish to isolate Parts to Tracks you can assign a Part to USB 1/2 and render just that single Part. Those decisions are yours to make.

MIDI Tracks cannot be MIXDOWN to audio - the MIDI must be routed to the MOXF hardware, it then responds by producing Audio, which you "return" to Cubase via USB 1/2 or USB 3/4. It is this returning audio that can be Exported using the Audio Mixdown feature.

Did you setup using the Yamaha MOXF6/MOXF8 Editor VST?
And very important: which version of Cubase are you using? In the latest version (Cubase Pro 8) you have a new "Render in Place" feature, if you are using AI you can use the FILE > EXPORT > AUDIO MIXDOWN feature

See also my responses in this thread:
VST INSTRUMENT SETUP

Recommended reading (Articles):
MOXF6/MOXF8 Editor VST
Moving a MOXF SONG into a Cubase Project

I have "set up using the editor VST" - but to be honest I don't really understand what I'm doing.

I am using Cubase AI7.

OK - lots to re-read and re-think about above.

I'll start working on building my understanding - and my skills and when I'm ready to pull out what little hair I have left. I'll ask the next series of questions!

Gumby