It only blocks most of the UV. The lamp itself is blasting a very intense amount of light, so that the circadian-sensitive mechanisms in your eyes are fooled into thinking you're outside on a bright summer day. The vitamin D is secondary to the main function of the lamp, which is to be incredibly bright in comparison to most indoor lights. The light-therapy lamp must be aimed at the eyes (even for blind people!). It just doesn't work as well if you can't see it directly. You would have to pump even more light into the room to get the same amount reflected off of other surfaces.

Also, the lamp could be putting out UVA and not much UVB, so it could tan without producing much vitamin D.

For just the vitamin D, I'd get a "5.0" UVB lamp in a bare-bulb fixture, designed and sold for indoor avian and reptile pets. Aim at arms and hands, and away from the eyes. They're about $15 per bulb, and should be replaced every 6 months, as the UVB output degrades over time. Afterward, they work perfectly fine as compact fluorescents, behind glass or under lampshades, for several more months, albeit shaped inconveniently for some fixtures.

A "10.0" bulb is for desert-species pets. The UV output is too high for the overall lamp brightness, and may damage your eyesight, as human pupils don't constrict enough. I suppose you could wear sunglasses, but that seems counterproductive to just buying a lower-UV lamp.

These "light therapy" lamps don't. They just supposedly improve mood from the exposure to bright light itself and don't claim to produce vitamin D.