You shouldn't have to. The Spot Color will print on a separate plate as intended for the varnish. If you are trying to limit your plates to just 4 Process Colors, then you will essentially be eliminating the fifth color ( varnish ). The job could be split into 2 separate prints; 1st print is the Process Color seps; 2nd print would be the solo varnish plate. But, you shouldn't have to do that ( 2 separate files of the same job ).
Select the path (object) for the Spot Color (varnish plate) with a selection tool and go to the the Attributes Panle and select Overpinrt Fill.
And make sure it is on top of the Image in the layers panel it does not have to be on its own layer just on top then you can print it or save it as a pdf as separation.
If you do not set it as Overprint AI will see as knocking out the image under it there will be no image so make sure it is set to overprint.
They are 2 bitmap files, so there is no "fill" to be assigned to O.P.
Thanks for the input guys.
What I was able to do was select the image in Illustrator, perform live trace, remove white background, fill with the image with the spot vanish color and then set it to overprint.
The goal was to get one pdf file that included the four color and spot varnish without anything being knocked out.