The AI does not fire STRIKE-tagged weapons like Reapers against frigates unless the weapon is also tagged USE_VS_FRIGATES or the ship is in exigent circumstances like being about to get blown up.
This is also just flat out wrong. The AI happily uses reapers vs frigates despite lacking the tag. This is the primary reason I never put reapers on frigates; the AI just wastes them when dueling other frigates at the start of the battle.
To clarify, this is *supposed* to be correct, but checking into it just now, there's a bug where the value for "whether it should use STRIKE weapons vs a frigate" isn't actually being checked. I've fixed this up.
There was also a Vanguard-specific bug where it would be more likely to fire torpedoes off when at high flux due to the ship having a right-click defensive system, which caused it to incorrectly assume the ship was in trouble if its flux was high. I've fixed this up as well.
Note that it will also be more willing to fire torpedoes vs frigates as the ship's hull level starts to drop - it doesn't have to be
particularly low for this; both hull level and the amount of ammo remaining factor into the decision.