The Strange wrench tracks both Kills with wrench and Sentry Kills. But, the item itself is only effected by Kills with the weapon, and the Sentry Kills are just tracked underneath it, much like Stange mediguns (not Kritzkreig) with their UberCharge being the main counter, and assists being the secondary counter.
Moving on, you would need the strange part: Robots Killed on a weapon, but it tracks kills with the weapon, not something it creates (like a sentry gun), as it would track only Melee kills. And, seeing that the PDA can not get any strange parts on it, there is no way to track sentry kills on MVM robots.
NOTE: If you are you using the Strange Gunslinger, it counts Sentry Kills as its main, then Melee Kills as its secondary.