Vanadium steel is an alloy used by the UNSC in the construction of its warships. Vanadium Steel is not as strong as Titanium-A alloy and so it is not used as armor plating, but rather in the making of essential and unarmored components. It likely is used as the basic plating for a ship, welded on over the TR Steel superstructure of the ship. Engine shields and baffles of destroyers are commonly made of vanadium steel.
Covenant Spy Probes are designed specifically to blend in perfectly with Vanadium Steel components so that they may fulfill their missions with ease. One Spy Probe in particular was effective enough to find Reach, alerting the Covenant and resulting in the Battle of Reach.