It's uncomfortable sweaty hell for the gunner, can't rotate at all, and the gunner can't see and relies on signals from a spotter. It is possible to use.
The first problem can be lessened by rotating vanguard gunner every minute or so.
The second is mitigated because we already aim the ship instead of the guns, and the front guns would usually be pointing forward anyway when in an anti-skiff spread pattern.
The third problem is a problem. Either the gunner relies on a spotter which lessens reaction times, or just keeps shooting every two seconds and hopes to hit. Just keep shooting isn't a bad idea due to the high rate of fire.
Sheer volume of fire should still make the vanguard much superior to 2 kettleguns despite the shitty mount. It shoots 1.5 times as often for the first minute or two, then 7.5 times as often afterwards. Fights between our corvettes can last 15 or 20 minutes so this is a huge improvement for most of the fight. The kettleguns rely mostly on luck and volume of fire to hit anyway so the accuracy flaws aren't that crippling. Even if it has a 1/4 fraction of the hit rate, the rate of fire will still make it advantageous.
Buying a vanguard is a wasteful idea with our current mount and prices (probable exception for a royal topaz vanguard), but it would be stupid to waste our free prototype vanguard any longer. We have to send it out at some point, can't keep waiting for things to be perfect. This goes for both the cannon and the viper, since we would have to keep Ophidian in dock an extra turn if we wanted to give it kettleguns instead of the vanguard and that would put us a turn behind in securing Three Captain's airspace and building a colony there. If we put the vanguard on it now we can send it out with the Royal Anguis and 6+ skiffs and secure TC airspace next turn.