Place the chocolate chips and butter into a bowl set over a pot of simmering water, and stir until melted. Add the peanut butter and vanilla, and stir again until homogenous. Remove the bowl from the heat, then add the pretzels. Using two spatulas, toss the pretzels to coat completely with the chocolate. Add the sugar to the bowl and toss again to coat the pretzels. Serve as is, or store in the refrigerator.