1.Create the VST Instrument

2.Set the output of a Midi track to the VST Instrument

3.Program its part with the Midi Track.

4.Now if other tracks are present , mute them.(Or solo the Midi VST Instrument Track)

5.Set the Left and Right locaters on each side of the part you want to turn into Audio.

6.Go under the File menu in Cubase to "Export" , then choose Audio.

7.Set the parameters here accordingly, and choose a location for the new Audio File.
You should choose the same folder you are using for the current cubase song.

8.Name the file , and make sure "Import to Pool" is checked.

9.Now hit "Save"

10.Now look in the Audio Pool of your song.You will see the new Audio File.Just drag it to a Audio track , and viola! You just turned the VST Instrument into a Audio Track.You can now delete the Audio Instrument ,or use a different sound!