@Troy: Thanks. This isn't a particularly easy game to pick up, and I like to help. On an unrelated note, let me know if you need any help with that tournament you're wanting to run. I'd be glad to be of assistance.
@Skavenknight: You seem to be making two arguments for why we should think BA Vindicators get to use a large blast; I'm going to call them the 'fluff' argument and the 'RAW' argument. I'll explain why they're both flawed and then suggest what I think a better (but still flawed) argument might look like.
The 'Fluff' argument: The fluff for the BA Vindicator is the same as the fluff for the SM Vindicator; therefore they should have the same rules.
There are a number of examples of vehicles/units/wargear that have the same fluff, but different rules. Take, for example, the Daemonhunter Assault Cannon. In the fluff, it is the exact same weapon as the SM (and BA) assault cannon. In the rules, however, it is Heavy 3 instead of Heavy 4 and doesn't have rending. Or, if you'd like an example involving a vehicle, take the Witchhunters' Rhino. I pay 50 points for a Rhino that doesn't come with smoke or a searchlight, despite the fact that, according to the fluff, it is the exact same vehicle. In short, we have ample evidence that fluff and rules do not always coincide.
The 'RAW' argument: The BA Vindicator has a Demolisher Cannon. The BA Demolisher Cannon is an ordinance weapon. The BRB says that 'unless their profile specifies otherwise, all ordinance blast weapons use the large blast marker.' (GW really needs an editor).
The problem with this argument is that the BRB refers to 'ordinance blast weapons', while the BA Vindicator doesn't have an ordinance blast weapon. It has an ordinance weapon, but not an ordinance blast weapon.
The 'RAI' argument: Ok, I concede that RAW doesn't support using the large blast and that fluff isn't particularly important when it comes to rules disputes, but I think that the Vindicator was intended to have an ordinance blast weapon and it therefore makes sense for you to allow me to use it as one. Considerations in my favor: there aren't any examples of GW nerfing a unit from one codex to the next, particularly not without changing its fluff. The BA Vindicator's cost is appropriate to having a large blast template; it is 30 points more expensive than the SM version, which accounts for it having the 'fast' property, and you'd expect the cost to be less if it was nerfed in this manner. The fluff describes it as having a blast-like effect. If it doesn't have a blast, it is basically playing the same roll as an assault cannon (except not as good), whereas if it has the blast, it is playing a roll that BA cannot otherwise fill.
I wouldn't expect this to win over any TOs, but I'd expect most reasonable opponents to let you have the blast in a friendly game.
If all else fails, you could try explaining that all Vindicators, large blast or no, are on the low end of the power curve and that they should be happy that you're taking a Vindicator at all. You could, after all, be taking a Predator.