The Vanguardian can be unlocked via crafting at the Resistor Crafting bench. This special workbench is located in the Luminopolis sub biome or at a Beacon Fixture in clubs.
If you want to unlock the Vanguardian without having to craft it, this class is also available in the Vanguardian pack and the Vanguardian Super pack.
These packs can be found in the Deals tab of the in-game store, come with extra goodies to help you fight crime in style, and support the Development Team and the creation of new content.