I wear varifocals as well.
With iron sights when I am plinking I just tip my head back a little and use the reading part of my glasses. When I am target shooting I have used a +0.5 pair of reading glasses worn over my normal glasses and use the middle part of the varifocal lens. I have progressed to using an iris mounted in front of my normal glasses as it really increases the depth of focus.
I also use a scoped pistol and a red dot sight with no trouble.
Diopter target rifle sights are giving me trouble as it is difficult to find a good part of the lens to look through and I finding distortion if I get it wrong.
rifle scopes don't give me any trouble.