This quest is completed at the same time as Render Unto Caesar.
You will a tip from Yes Man about Mr. House's hidden bunker beneath the Legion base. If you do this right away, you should still have Caesar's Mark and be able to get in without the Legion being hostile. Speaking to Caesar will allow you access to the bunker, and also give you the Platinum Chip if you didn't get it from Benny previously. Upon getting to the bunker, you will find out that it is actually a Vault.
The Vault is full of robots, from Turrets to Protectrons to Sentry Bots. You need to find the "Computer Room Door", which is found up some stairs in the far west. Inside, you'll find a Securitron Operations Console, which you need to place the Platinum Chip into. This will upgrade the Securitron army.
You can also destroy the generators to destroy the army. This will flood the vault with radiation so you will have to get out quickly.
Return to Yes Man at the Lucky 38.