I don't know the answer for sure, but my guess is that "Various Artists" is a tag that many music databases use for multi-artist compilations. This is a fixed tag that is in the Artist field. The "(Multiple Artists)" value, however, is a dynamic/automatic value used by MC to designate the "Album Artist" of a compilation that is automatically set when MC detects an album has multiple artists.
So, how to you consolidate your "Various Artists" entry into the automatic "(Multiple Artists)" one? I've done it in he past, but its been a while, but here's what I think:
1. Make sure the Album field is the same for for all of the tracks in the album.
2. Overwrite the Artist field (not the Album Artists field, that one is automatic) with the artist for each track.
3. After all the tracks are completed MC should detect that they are multiple tracks in a multi-artist compilation and automatically fill in the Album Artist field. I don't know if you need to do something to refresh this, though.
Good luck.