Multi-vector assault mode is the ability of a Prometheus-class starship to divide into three independent sections, each able to fight on its own.
The USS Prometheus was an experimental ship built with advanced tactical systems, including the ability to split into three separately armed hulls that could attack a target from different directions. The design was kept secret during its early trials.
Romulans hijacked the Prometheus during testing, and the only crew left to resist were holograms. The Voyager EMH, transmitted across the galaxy through an alien relay network, worked with the ship's EMH Mark II to retake it, triggering the assault mode to defeat Romulan attackers.