The title is the key to this short poem. "Vicarious" means experiencing something through someone or something else, and that is exactly what the speaker does. Instead of pursuing a person, they look at pictures of hands and let those images stand in for the real thing. The word "tend" makes this desire sound like a small fire or a garden, something that needs steady, patient care.
The second image is stranger. A tar merchant "greedy for the night" suggests someone who wants darkness the way a dealer wants stock. Tar is black, sticky and thick, so the comparison gives the speaker's longing a heavy, clinging quality. It also hints that this looking happens in private, after dark, and that the hunger is bigger than the modest pictures can satisfy.
The mood is quiet, intimate and a little lonely. The speaker isn't dramatic, but the wanting is clearly strong, and the poem leaves it unresolved.