You can create a new mask and use this for both
+ ifnotempty(year,' - ')
+ 'Disc ' + discno
+ ifnotempty(pad(trackno,2),' ')
because the AlbumArtist will be the artist you want for single artist releases and "Various Artist" for Various Artist releases, and as far as I can see no other differences are reuired by you.
However if did need to handle Various Artist compilations different to Single Artist Compilations and Single Artist non compilations for your compilation rename mask you could use the following logic for your compilation rename mask
#Put Various Artist Compilation mask here
#Put Single artist Compilation mask here