Alpha Shield + Vengeance is a really common tactic in high-level BLS, and it's quite difficult to counter compared to many other finishing moves. If it's sufficiently powerful, you can ONLY counter it with Alpha Shield (or Blast Shield, but no one uses Blast Shield in BLS).
If you try to deploy Repair Drone in preparation, you'll find that your Battleship is KO'ed by the blast, and appears back in your Yellow Star partially healed.
Example of how it currently works:
Shield + Hull strength: 14,000
Repair Drone heal amount: 7,000
Vengeance blast: 18,000
The Vengeance blast KO's you and sets your HP to 1, but will appear in your YS with a final HP of 7001.
To provide a counter to this nearly ubiquitous finisher move, a simple change could be that the net damage you receive equals (Vengeance DMG - Shield + Hull HP - Repair Drone heal amount). I believe this will be way more balanced since you will still not survive if you're weakened or if your Repair Drone isn't strong enough, but it gives Passive Shield users a fighting chance.
Example 1 of my proposed change:
Shield + Hull strength: 14,000
Repair Drone heal amount: 7,000
Vengeance blast: 18,000
The Vengeance blast is partially offset by the Repair Drone, dealing (18,000 - 7,000) = 11,000 damage. Your battleship will survive the blast with 3,000 HP.
Example 2 of my proposed change:
Shield + Hull strength:** 10,000**
Repair Drone heal amount: 7,000
Vengeance blast: 18,000
The Vengeance blast is still partially offset and deals 11,000 damage, but your battleship is not strong enough. Your battleship is KO'ed in the blast and appears in your YS with 1 HP.