why would you ever use a vanquisher cannon?

[Bunnahabhain]

The alternate profile negates the AP1 that is perhaps their biggest bonus over something like a Vanq.

It does seem like I might be thinking of the rules wrong for ordinance--it's been a long long time since I ran any in my Guard. I thought ordinance could not shoot if you moved? Or is it that you can't fire defensive weapons if you fire ordinance when moving? If I am indeed wrong, then my opinion of Medusas just improved greatly.

Yes, you and others are indeed wrong.

From the freely available from GW IG reference sheet, so I can repeat it here for the avoidance of doubt.
Profile 1. 36", S10, Ap2, large blast, Ordnance

Profile 2 48" S10, AP1, small blast, 2d6 added together

There are NO restrictions on moving and firing normal(i.e. not barrage) ordnance weapons over and above a normal weapon of their strength.
You may not fire ordnance and any other weapon, unless you have a special rule, which Russes do, but the vanquisher cannon isn't ordnance anyway...

[MyNameDidntFit]

Huh. I had Medusas entirely backwards in my head. Goes to show how often I even think about artillery batteries beyond my darling Griffons.

Well, you learn something new every day.

[Getz]

Nah, Getz is just wrong. They're AP1.

Sorry, my bad - could have sworn they were AP2...