I simply cannot imagine how one could have 'replaced the glasses' as you have mentioned. Bulletproof glass is atleast 1.5 inches thick and putting back normal glasses is as big a modification job as installing bullet proof glasses in the first place!
Fuel consumption on bullet proof vehicles is not an issue, but is normally very low compared to regular versions of the vehicle due to its high weight! If such a vehicle were bullet proof, it would weigh atleast 4.5 tonnes!
Or have I understood you wrong, and you meant bullet proof in terms of quality 
?
