Faction design/theme is constant CHOOSING between defensive and offensive options.
AEC vs Bofors?
Hammer vs Anvil?
How has that eluded you all these years?
If you want to go with faction theme, that bonus being both defensive and offensive makes perfect sense and given how the ability was up until now, it perfectly enhances its flavor.
For that argument to make sense, surely you would need to choose between two separate, and exclusive MG variants, or at least have the Vickers receive a different vet bonus/active ability based on the UKF player's choice of teching? UKF "design" was the choice between "Defensive" and "Offensive", not an unit doing both at the same time.
Though UKF faction design has moved away from the "choice" between Offensive and Defensive options more recently, with the AEC and Bofors becoming non-exclusive as of the balance patch.
Either way, Assuming implementation isnt impossible, cannibalising the Bofors' "Suppressive volley" code, or as Vipper suggests; the Centaur's vector-targeted strafe ability (Along with using one of their projectiles with edited properties) might provide an avenue into making this ability a reality.
Alternately, don't a few of the offmap strafes provide an "aoe damage and suppression" effect without the use of projectiles? Theoretically you could simply have the Vickers fire harmlessly into the air, and then "call down" a small AOE suppression strafe to achieve the same effect, right?
Obviously this assumes the Balance team is interested in making the time investment required to give the Vickers a brand new Veterancy ability, with the subsequent balancing it will require. This isnt hugely likely, but the suggestion is there for them if they do think it has merit.
I'd love to try and implement it myself, to save them the effort, but I have basically no experience with the CoH2 modding tools (And my programming time is currently in-use for personal projects).