According to Hubert Vos' Wikipedia entry, he sometimes made copies of his paintings while living in Asia -- one for his client, and one to take with him. Evidence of that is suggested in "Harmonie" (1910), as can be seen below.
In the center-left a painting is depicted, near furniture and vases from Asia. The Wikipedia entry mentions that Vos lived and worked for several years in China and Korea, so clearly he brought from there various objets d'art along with some duplicate paintings.
The painting-within-a-painting is clearly this one.