class vertexStruct
{
public:
vertexStruct(int fvf);

private: 
int fvf; 
int stride(); 

  // get Offset returns the offset in the 'struct' to this component. e.g. position or color
  int getOffset(int fvfComponent)
  {
    // would do math involving the fvf member here
  }

  // returns true if fvf contains fvfComponent
  bool contains(int fvfComponent)
  {
    return fvf&fvfComponent;
  }

};


Then your function that builds your sphere for example: 

buildSphere(float* modelData, vertexStruct vs, int vertexCount)
{
  for(int i=0; i<vertexCount; i++)
  {
    if(vs.contains(D3DFVF_POSITION))
    {
      modelData[i*modelData.stride+getOffset(D3DFVF_POSITION)]=1.0f; // x position
     modelData[i*modelData.stride+getOffset(D3DFVF_POSITION)]=1.0f; // y position
     modelData[i*modelData.stride+getOffset(D3DFVF_POSITION)]=1.0f; // z position
    }
  }
}