In a "linearly ordered set" such as the real numbers, rational number, integers, .... you can identify one element a "0", elements to one side as "positive", elements on the other side as "negative". But vectors are not "linearly ordered" so there is no way to declare one set as "negative" and another as "positive". Of course, given a vector you multiply that vector by the scalar -1 and get the "negative" of the original vector. In that sense the "negative" of <a, b> is <-a, -b>.