GLES20: vprog textures are used, but not supported.

I've built a game in Unity 5.0.1p1 (a patch release for fixing a vertex color issue) with custom shaders, in which texture lookups textures are used in the vertex shader. It works correctly in the editor. Deploying it through Xcode works on an iPad Air, but not on an iPhone 4S.

The specific error I'm getting is in the title, which repeats several times, like so:

 Note: Creation of internal variant of shader 'Custom/ShaderName' failed.
 GLES20: vprog textures are used, but not supported.
 
  
 (Filename: /Users/builduser/buildslave/unity/build/Runtime/GfxDevice/opengles/GlslGpuProgramGLES.cpp Line: 332)

After this it will show a random other built-in shader that isn't supported either, presumably due to the other shader failing to be created.

 WARNING: Shader Unsupported: 'UI/Default Font' - All passes removed

I'll assume that "vprog" means the vertex shader here, and I know OpenGL ES 2.0 is used, which should support vertex texture lookups. All the used textures have a power of 2 resolution.

This post mentions that

you have to reference the texture on the fragment shader as well as on the vertex shader, even if you don’t require access to the texture on the fragment shader.

If this is correct, is there a way to see the compiled OpenGL ES 2.0 shader that is made for the iPhone, to check whether this is the case?