It isn't obvious to me what you mean.

Giving us a little more detail about the intent of the code would help.

On a side note your loop is incorrect, it increments the outer iterator past the end of the vector. A correct version is:

for (std::vector<char>::iterator it = keys.begin() ; it < keys.end(); ++it)
{
      // ...
}

Maybe you want to loop one more time than you have elements in the vector. In which case the cleaner solution might be to explicitly do that:

// You could explain why you want the extra iteration here...
int limit = keys.size() + 1;
for (std::vector<char>::size_type i = 0 ; i < limit ; ++i)
{
      // ...
}

Have you tried comparing keys.size()?

Note that you must be careful structurally modifying a vector when you are iterating over it, it is easy to end up with invalid iterators and/or accessing elements incorrectly, or stepping outside the bounds. One option might be to remove excessive elements before entering the loop (e.g. using erase()), so you don't have to worry about this condition in the loop body.