At our unit, we get a lot of VFR (usually University Air Squadron) wanting to do a VFR ILS approach, so we generally say:
"Vectors for an ILS approach rwy xx all headings and levels are advisory, report if at any time you are unable to maintain VFR"
That way they know it's VFR, they follow my vectors, if they can't mainatin VFR then they tel me, simple really