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 line "Uselessly vacant, abandoned beside the easel" talks about the chair that is no longer used by the painter, you can infer that because of the word abandoned, which means that it was occupied once, "uselessly vacant" expresses that there is no point in having an empty chair that nobody is going to use.
