Bonesaw is similar to mechanic’s wrench. They are both single handed melee tool-items. And both will let you do you damage, potentially. Vincent's weapon has an updated, better version of the Kukri effect. If he wants to, he can do an extra damage, trigger bandages or Jessica Hyde, then slash again with a fresh 'on the mend'. Kind of a dark mirror of the meat cleaver.
The secondary effect feels like an emergency button; there are very few effects in the game that can reduce that much damage on someone, especially if you’re healing people steadily to give them on the mend. Combining that test with premonition, neither rain nor snow, or other downside mitigators is best. If your allies have trauma, it should be because of 'in the thick of it'.
Since it’s a tool, it can go back in your Tool Belt when there’s nobody around to stab. Situationally very useful, which is the sweet spot for a signature card.