The chin mounted S-Cannon is a great option for your Vypers. To keep my Vypers cheap I usually run dual cannons. Here's my solution for how to mount them without having to use expensive pewter Jetbike bitz, which I'm not even sure will work. Plus, these are removable.
The Vyper frame comes with two cannons, one on the weapons sprue and one on the Vyper sprue. The one we're going to use is the one on the Vyper sprue. It would probably work with the other, but it is built differently and I haven't tried using it.
Here you can see in this terribly blurry photo (I took it on my phone, I was too lazy to get out my tripod and camera) what I've done. First thing you need to do is clip or saw off the ammo feed belt. Next thing to do is to saw off the back bulbous thing. Next, glue the bulbous thing at approximately a 20˚ angle on the rounded part. If you just leave it normal it won't fit underneath the Vyper. Finally, glue the ammo feed belt underneath it all. Lastly, fill in the circular hole with some greenstuff, and magnetize it for removal. Not too difficult!