It would be best to clamp it up using the swing arm bolt and nut in case the bushings have poor alignment, eccentricity, centering or the mating surfaces are canted. While you are likely good to go employing the vise method you still may not be. All you need is to find a spacer to take up the space the motorcycle frame metal would occupy and install it under the bolt head and/or nut. Cover all the bases is my thinking, especially if it tightens up a lot when you install it to the bike and torque the nut. Maybe you could find a socket or axle spacer to use as a tool?