android-3 -> Official Android 1.5 system images
android-4 -> Official Android 1.6 system images
android-5 -> Official Android 2.0 system images
android-6 -> Official Android 2.0.1 system images
android-7 -> Official Android 2.1 system images
android-8 -> Official Android 2.2 system images
android-9 -> Official Android 2.3 system images
android-10 -> Official Android 2.3.3 system images
android-14 -> Official Android 4.0 system images

Currently in gles20 branch's project.porperties, android-10 is required, which means 2.0 games can only run on android 2.3.3 or higher devices.
THIS IS UNACCEPTABLE.

Finally I find that gles20 depends on android-9 and higher, coz CCEGLView includes EGL/egl.h, while android-8 has no EGL headers.
We depends EGL to get only 3 APIs,

1. glGenVertexArraysOES
2. glBindVertexArrayOES
3. glDeleteVertexArraysOES
   In CCGL.h, these 3 APIs are redefined
   

   #define glDeleteVertexArrays        glDeleteVertexArraysOES
   #define glGenVertexArrays           glGenVertexArraysOES
   #define glBindVertexArray           glBindVertexArrayOES
   

   
   And finally they are invoked in CCParticleSystemQuad.cpp & CCTextureAtlas.cpp, with a pre-condition that marco CC_TEXTURE_ATLAS_USE_VAO 1
   CC_TEXTURE_ATLAS_USE_VAO marco is defined in ccConfig.h
   

   #ifndef CC_TEXTURE_ATLAS_USE_VAO
       #if (CC_TARGET_PLATFORM == CC_PLATFORM_IOS) || (CC_TARGET_PLATFORM == CC_PLATFORM_WIN32)
           #define CC_TEXTURE_ATLAS_USE_VAO 1
       #else
           /* Some android devices cannot support VAO very well, so we disable it by default for android platform. */
           #define CC_TEXTURE_ATLAS_USE_VAO 0
       #endif
   #endif
   

   
   So it means on android platform CC_TEXTURE_ATLAS_USE_VAO 0, it's disabled.

OK, so if your game is very performance-hungry, you can enable CC_TEXTURE_ATLAS_USE_VAO to run it faster, but your game is limited to android 2.3 and higher devices,
By default, we should disable CC_TEXTURE_ATLAS_USE_VAO to adapt games on wider range.