A challenge coin must have the rank of the commander issuing it. That's the challenge part: If you throw down a CC in a bar, everyone has to produce a coin. If they can't, they buy the round. If everyone has a coin, then the lowest rank shown buys.
I'd replace the "vega campaign" with the rank of the ship commander, and probably his name. The backside should feature the ship's insignia; not just a picture of it.