Also do not forget that even if you find a valid address, chances are much more than 99.99 % certain it is a wallet with no coins (generated by another vanitygen).
You are always finding valid addresses with VanityGen, however if you set VanityGen to find a solution for a full length address you would obviously make sure that address you are bruteforcing held Bitcoins.
Its logical to brute force against the largest address because its illogical to search for any other address due to time and energy requirements.