If you might need to hold more than INT_MAX items in your vector, use size_t. In most cases, it doesn’t really matter, but I use size_t just to make the warning go away.
Better yet, use iterators:
for( auto it = v.begin(); it != v.end(); ++it )
(If your compiler doesn’t support C++11, use std::vector<whatever>::iterator in place of auto)
C++11 also makes choosing the best index type easier (in case you use the index in some computation, not just for subscripting v):
for( decltype(v.size()) i = 0; i < v.size(); ++i )