As the Vibro Cannon is not 'fired' as a Snap Shot nor has a special rule saying it can not fire as a Snap Shot (like Blast and Large Blast p33, Ordnance p51, or Template p52) it comes right back to what resolving the shot in the Hard to Hit rule means.Some weapon types, such as Template and Ordnance, or those that have certain special rules, such as Blast, cannot be fired as Snap Shots. It is important to note that any shooting attack that does not use a Ballistic Skill - such as the Necron Monolith's portal of exile - cannot be 'fired' as a Snap Shot.