A Vindicator comes with a Demolisher Cannon - S10 AP2 Large Blast, Ordnance and a Storm Bolter - S4 AP5 Assault 2. I had always assumed that you could fire your big juicy blast and your Storm Bolter in the same turn. However, in the first round of Open War, Chris pointed out that I couldn't shoot my Storm Bolter if I'd fired my Demolisher Cannon. Of course he was right and I'd been using my Vindicator incorrectly for the last 6 months. From page 58 of the Big Rule Book - "Firing a massive ordnance weapon requires the attention of all of the gunners of the vehicle, so no other weapons may be fired that turn (not even defensive weapons!)"
So, there we go. I guess it just leave the question, why do you get a Storm Bolter on the Vindicator?