Playing "Vanquish Confirmed" means playing some kind of Team Deathmatch. Each team has to collect 50 points altogether to win the match.
After a plant or zombie has been shot down, it will leave a green (plant) or purple (zombie) glowing orb. Only those killed opponents whose orbs are collected by the opposite team will count as "confirmed" kills.
Said that, to get the achievement you have to collect a total of 50 of those "crazy" orbs. It doesn't matter if the enemy has been shot down by yourself, as long as it is you to collect his orb, it will count for this achievement. The counter is cumulative, so this has not to be done in one single session.
Happy hunting!
Thanks to tornprince2012 for pointing out that orbs from fallen teammates don't count.