There’s no need to bounce or convert. If you follow Export> Audio Mixdown, then audio from your VSTi tracks will be included as part of the generated audio file. If the MIDI tracks are routed to a VSTi, then they will also be included.
Basically, if you can hear it on playback through the output channel you choose for the export, it will be included.
However there is another school of thought that says while it is not necessary to render tracks before mixing it is wise to do so. The idea is that this makes it more likely you can access the project in the future. Say you want to add some parts and remix the project 8 years later. If the VSTi you used no longer runs under that version of Cubase/OS you’ve lost that audio. Or even more extreme what if you’ve switched to Tesla’s SuperDAW3? You can always use the rendered audio if it exists - anything else is less certain. This same logic would also call for rendering vocals with reverb & compression etc.
Having said that, I usually don’t personally render audio for future compatibility. I may regret that someday, but so far so good (knock on wood). Others do it religiously - a pro studio for example would be smart to do this.