Right now, Ad Mech feel too dependent from the nova canon and their torpedo launcher ships are too weak compared to others because they only use basic torpedoes. I think that giving them vortex torpedoes could help with that and feel ok lore wise since they are supposed to be the one who use high tech weaponry in the Imperium.
Also, the Ark Mechanicus could use a torpedo launcher. Maybe not 8 torpedoes like the Retribution since it's also have a Nova canon but 6 could be good. Right now, it's too weak and it's better to take 2 Tyrant for doubling the use of Nova canon will keeping a good Macro dps.
As for the effect of the vortex torpedo, I don't know. Maybe it could work like eldar torpedo and ignore armor or have double the critical chance so it's better to destroy systems. Probably only one use since they are supposed to be really powerful (and they are supposed to be the rarest type of torpedo).
Of course, price of the ships might be changing in the process since they become more powerful with better torpedoes.