Starting in an earlier version, I had a wall with some vines placed at the top (on stone bricks), which then grew down over a window (of glass blocks). Today with version 1.2.5 I noticed a gap and tried to fix it, and found that I could not place vines on a glass block. (Also, when I broke a vine growing on glass, the vines below it broke as unsupported.)
For all I know, this is an intentional change, but I haven't heard that it is, so I'd like to draw your attention to it in case it is a bug. I feel that vines ought to at least be able to grow down past glass blocks, even if they cannot be placed unsupported on them.