The Vanquisher is a great piece of engineering, but it wasn't as great as advertised.
In particular, unless Natalya has special stats for the main gun, it was very weak and couldn't really destroy much in my humble opinion. I believe that was the main weakness of the Vanquisher. Of course, this thing stood its ground against a size 9 gun and missiles every turn and finished the game with two hit points so.
Size 9 guns are "special stats" too, ya know. Technically you're supposed to cap off at 5 (although that might have changed in 2010)
Anyways, this wasn't a problem about the vanquisher sucking. As the commanding officer in this battle, I focused pretty much my entire defensive strategy around crippling and outranging the Vanquisher so that I could outrange and destroy it ASAFP. It went down because I was absolutely terrified of it, and dedicated all of the Trattorian armored resources towards squashing it down.
Of course, it helped that the Trattorian SHDT was pretty much designed as an extreme tank-killer - there's no way I'd hit anything smaller than a tank with the UR of that main gun. Plus the extreme range meant that I could target them waaaaay before they could target me, so I could just blow the treads and keep em there.