I know I'm a few years late. But I stopped playing R for a long time in the middle, and IG is the very last faction I started playing.
So the Vanquisher upgrade is supposed to be the anti-vehicle option, and the plasma cannon is the short-range anti-infantry AoE option.
At first I bought the Vanq for dealing with enemy vehicles. But it didn't give me any appreciable results. My Vanq-equipped LR was driving around chasing Fexes, and doing piddly damage.
So I bought the plasma upgrade, which is incidentally the same price....
Anti-infantry: Friggin' godly. Rapid-fire AoE anti-infantry dps! The short range hardly matters, because your biggest anti-veh infantry threat will always be the melee troops (Bloodletters, Ogryns, Stealers) or the short-range anti-veh troops (Kasyrkins). The plasma cannon is perfect for gibbing them single-handedly while your tank backs off in reverse. Saved my tank's skin more than once.
Anti-vehicle: Friggin' godly. A dreadnought was chasing my tank as it was backing away. 1 volley of several plasma shots reduced its lifebar by around 50%. Short-range? Not really. I win shootouts vs enemy tanks half a screen away, with the plasma cannon upgrade. Do I actually need sniper-like range? Not really. I want my tank's machine guns and lascannon to be useful too, after all.
Is my impression correct and the Vanq upgrade is just crap?