I use an A2Z adapter I bought on Ebay for about $30. It works well although it took a little filing to fit my Cannondale. The others are correct that you don't really need a disc on the rear, the rear wheel is not really used for braking, so v-brakes are fine. I only use the rear brake as a backup in case the front fails.

I only got it because I got a disc-only wheelset and couldn't use v-brakes.