Why use glacial acetic acid in bromination of anisole?
[...] bromine is not reactive enough to brominate an aromatic without FeBr3


That's true for benzene, but the methoxy-group in anisole isn't just o/p-directing. 
It is activating as well. Actually, it's that much activating that anisole will react with bromine even without the presence of a Lewis acid.

So, why glacial acetic acid? It's a polar solvent and thus nice to stabilize the Wheland intermediates. Moreover, it's easily removed in aqueous workup.

Edit 1
Does the acetic acid react with the (mostly undissociated) acetic acid? At least a polarization of bromine ($\ce{Br^{\delta+}-Br^{\delta-}...HOAc}$) is conceivable and I remember that $\ce{BrOAc}$ has been postulated as the electrophilic species. 

Edit 2
I remembered some brominations on dimethoxy-substituted hydroxymethylnaphthalines I did back in the days. The reaction was performed at 60 °C in $\ce{CHCl3}$ in the presence of about 3 equivalents of anhydrous sodium acetate. Excess of bromine was reduced by addition of aqueous sodium sulfite. The yield was in the 85% range and I don't find any notes on dibrominated products.

Edit3


  And with excess Bromine you would (theoretically) react all of the anisole?


Pretty much, with 4-bromoanisole as the main product.