The connotative meaning of the word vacant in this line from the poem is that the word implies that the chair is not only empty, but unnecessary.
Explanation:
The definition of the chair being both empty and unnecessary can be inferred by the use of the expressions "abandoned", that means that it was occupied once and is no longer used, "uselessly vacant" expresses that there is no point in having an empty chair that nobody is going to use again. These expression are quite similar to the ones in the first definition.
