How can I deform a rectangle to involve a pillar?

I am trying to make a rectangle with 18 divisions involve another rectangle.

A picture from the rectangle:

My best result:

The code:

edit: I have improve the code, but it doesn't work well yet.

 int i = 0;
 int triangleCount = 0;
 for (int k = 0; k < vertices.Length; k++)
     {
         if (i == 2) // two triangles
         {
             i = 0;
             triangleCount++;
         }
         float newX = R * Mathf.Cos(degree * triangleCount * Mathf.Deg2Rad);
         float newZ = R * Mathf.Sin(degree * triangleCount * Mathf.Deg2Rad);

         Vector3 vertex = vertices[k];

         vertex.x += (origin.x - vertex.x) + newX;
         vertex.z += (origin.z - vertex.z) + newZ;
         vertices[k] = vertex;

        Vector3 normal = normals[k];
        normal.x += (origin.x - vertex.x) + newX;
        normal.z += (origin.z - vertex.z) + newZ;
         normals[k] = normal;

         i++;
     }

Is there any way to do this?