There are lots of sleek subway shaped tiles out there. I wouldn't do the granite BS either. Give the walls an extra coat or two of paint, and don't be in a rush. Of course it took me ~ 3 yrs to find the 'perfect' BS to go with my butterfly blue granite (I'm not recommending waiting that long! LOL)
granite is actually more the colour of the glass tiles than it appears in this photo...:>)

Plain Starfire sheet glass over your paint would give you the minimalist look and preserve the paint color as well. You don't want the regular greenish tint glass though. You want the low iron super clear.

The manager at my local Benjamin Moore said to apply two coats of primer and then two coats of paint for optimal protection for the walls around the vanity. I'm going to give it a go without a backsplash. Think it will work??

Yes, if you don't splash a lot of water on the back wall. Or if you do, just dry it off when you're done.