Nail varnish remover
Acetone, dip a cotton bud into the bottle and swizzle it into the varnish, then check for transfer.
The problem you do not want to have is to take a thin line of nail varnish and turn it into a diluted spread out stain.
I remember years ago attending a advance stain removal course that Recommended creating a barrier around a stain to prevent the stain bleeding out into the surrounding fabric. Get a bar of fairy laundry soap cut it to a point and draw around the stain then theoretically any solution applied would stay within the soap barrier . You need a way of delivering the acetone directly onto the varnish with as little hitting the surrounding fabric.
Is it on a seated cushion? can you get to the back of the fabric?.