Colin, I guess vlookup is used just to generate a value for every entry, that can be counted vith another formula... As you noticed, vlookup does not count, just returns a existing value from the selected range...
Aztiguen, you can set formulas to count by a time range, and the count will stop if the date when entries are added is out of your selected time range. This way you can count employees for every month and the solution is simple. If you can upload a sample worksheet it will be easier.