I'm intrigued... really, I'm surprised, when I first heard of WebGL I had assumed that texture access in the vertex shader would not be supported.
Well after a quick two minutes of reading it seems that Chrome and Firefox are not holding firm to the OpenGL E.S. 2.0. spec. I guess you can do it.
Have you tried printing what you are putting into the texture out to a text file as well so you can see if the data makes sense? That's usually how I debug.