Not quite... Rather, it is another template argument when you create the vector. The beauty of templates (ignoring really cool template metaprogramming) is that you can create user customizable code that is standard (term used loosely, but it still holds in the strict sense as well).
Look at SGI's documentation (my favorite source for STL docs) here: http://www.sgi.com/tech/stl/stl_index_cat.html
The section at the bottom is... Drum roll, please.... Allocators.