Here's an idea,
Put some items that aren't usually in vote shop for a month, maybe ICC heroic drops or TOC drops, just something that players with full wrathful or end game gear are willing to buy.
The idea is, make those special items very expensive, for example 150~200 vote points. This way players who are voting regularly can spend all their vp once every couple of months on an item that they would actually use.
Look at my vote points, I have 450 Vote points on this account, and I can't use them on anything, I know voting is to help the server, which is why I'm still voting in the first place, but at the same time I would like to spend those VP on something useful that is hard to obtain. I'm not gonna waste those VP on mats that I can farm easily or buy from AH.
If you want players to keep voting regularly, put something in vote shop that they will strive to get, keep the element of surprise, for example one month put Death's Verdict, the next month Deathbringer's Will, then the next month maybe a shirt with stats or a tabad, the shirt and tabard don't need to have the best stats ever, just something that is hard to get in the game will be fine. And afterall only few players will be able to buy those special items anyway.