All I want to do is to check whether an element exists in the vector or not, so I can deal with each case.
if ( item_present )
do_this();
else
do_that(
Bear in mind that, if you're going to be doing a lot of lookups, there are STL containers that are better for that. I don't know what your application is, but associative containers like std::map may be worth considering.
std::vector is the container of choice unless you have a reason for another, and lookups by value can be such a reason.