Virtually all aftermarket windshield makers drill 3 1/4" holes - for the straight style vent. If you are replacing "Angle style" vents, my suggestion is to buy a 3" sanding for your drill - about a $5.00 purchase - and enlarge the holes 1/16" to make the straight vents work. It is not hard to do.

Another method is to find a drinking glass with a little taper, the right diameter. Wrap it with sand paper and enlarge the hole by twirling the glass, sanding away until the new vent fits. This, by the way, is a very good method to finish enlarging the hole as it makes it perfectly round.