Can you post the entire vertex shader source? If you have a Cesium app with just the tileset and once the tileset is loaded, place a break point here:
https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/Scene.js#L2325
Then, in the console, output the vertex shader text:
commands[j].shaderProgram._vertexShaderText
Thanks,
Dan