Hi,
I'm sorry, I should have made it clear that i meant ortho and para relative to the vinylic group that your attacking with the HBr. You have put the positive charges where they are supposed to go, good work.
You forgot one resonance form where the methoxy oxygen octet resonance stabilizes the carbocation at the para position.
You are correct in thinking that if you put the bromide down on the benzylic position the compound would be neutral and the mechanism would be complete. In my opinion, this is the correct answer.
You can try to put the bromide ion down at the other points on the ring, but you will find that you cannot re-establish aromaticity if you do that.
I think when your teacher said "use what you know about electrophilic aromatic substitution" your teacher meant for you to to draw the resonance structures. If you draw them the same way with the nitro group present, you will see that the cation locates itself geminal to the nitro...this will slow your HBr attack on the double bond down, possibly even lead to no reaction on the nitro containing compound.
Is this making sense? You said you were confused, but you actually seem to have a really good grasp of this stuff.