I wouldn't say it "will not work properly", exactly. Less than ideal, but will definitely work. I completed the 50yd challenge (22lr, 5 groups on the same target sheet, .5" or less at 50yds) with a Vari-X III 2.5-8 mounted on my CZ452 Varmint. You just have to be aware of the parallax effect and watch for it, IMO.
So just how do you watch for the parallax effect and what do you do about it?