First, he sets the scene: he often sits on his couch, kind of feeling blah about life, with no great thoughts and sights. Sometimes his mind is empty and "vacant," like a bored teenager sitting on the sofa after school and trying to decide what to do. At other times he feels "pensive," which means he thinks kind-of-sad thoughts. You can’t be both "vacant" and "pensive" because one means "not thinking," and the other means "thinking while feeling blue." But he groups the two experiences together because both are vaguely unpleasant and dissatisfying.