def vertex_position(indexed_triangle, vertex_index):
    """
    Position of a specific vertex in an indexed triangle.

    :param indexed_triangle: List of three vertex indices.
    :param vertex_index: Vertex index to look for in the triangle.
    :return: 0, 1 or 2 depending on whether the vertex is the first, second or third vertex in the triangle.
        Returns None if the vertex does not exist in the triangle.
    """
    for i in range(3):
        if indexed_triangle[i] == vertex_index:
            return i
    return None