upper_bound

I'll show you upper_bound(v.begin(), v.end(), x) to find the first element strictly greater than x. It returns an iterator to that element or v.end() if none exists. You can count occurrences in a sorted vector by subtracting bounds: upper_bound(v.begin(), v.end(), x) - lower_bound(v.begin(), v.end(), x) gives the frequency of x.

The difference between upper and lower bounds identifies all elements equal to x. Both functions require sorted ranges and run in $O(\log n)$ time using binary search.