He probably sees in the dark, well enough to find victims, but not well enough to exist comfortably. What I think about is the strange fact that when you become a vampire, you're expected just to "be a vampire", without needing to live someplace except a crypt, or to have interests or other needs. How does he pass the time in the Tower Room? Well, he doesn't, really, he goes out for constitutionals. I can imagine Joshua closing the Tower Room door on him, with Barnabas thinking, "Well, what now? There aren't even any magazines in here."