The Vi'dar is predominantly used to fight off Borg Tactical Probes - the Borg hostiles. Vi'dar events and missions may be available for players of different levels, however, to build the ship itself you'd need a Shipyard of level 25 or above.
Vi'dar blueprints can be won through various Borg-themed events. Additionally, Vi'dar Packs may be available in the Store, providing half or the full amount of blueprints needed to build the ship.
For more information, please check the FAQ on our blog.