actually, vac plates are not universal, nor are skimmer diameters. You need to get the proper one for your make and model of skimmer. For example Hayward has about 7 different modes and Pentair has at least that many also, if not more.
For example:
http://www.nextag.com/skimmer-vacuum-plate/shop-html
http://www.epoolshop.com/skimmervacuumplates.aspx
http://www.thefind.com/garden/info-skimmer-vacuum-plate (this website has 5 pages of vac plates!)