Three short lines, and a small scene comes into focus: someone worn from the road, searching for a place to stop. The title, "Vacancies," sounds like the sign outside a motel, and the poem seems to hang on that word. The speaker, or the person being described, wears "the garb and gait of travel," so tired clothes and a tired walk give away a life spent moving.
The last line, "sign busy till late," is where things get interesting. It reads like a phrase from a lit sign, yet "busy" is an odd word for it. The sign is active, glowing, maybe flickering, while the traveler has nowhere certain to sit. Judging by the wording, there is a gap between an invitation and actual rest.
The mood is quiet and a little lonely, but not despairing. It feels like the tired, waiting stretch of evening when you're on the move and haven't yet settled. The poem leaves the ending open, so we never learn whether the traveler finds a place.