I did not read the book of M. Jean des Cars which you are talking about. But I guess it is a novel. The authors of historical novels may allow themselves to everything. They mix fiction and reality and attribute to real personages some adventures totally invented (A l'instar de notre Alexandre Dumas, maître du genre). The Vetsera were too small for nobility that the Head of the House of Bragança thought to marry the little baroness who was only 16 years old and came from Cairo...
Besides, the Vetsera were not received by the Court of Vienna, at the Hofburg Palace...
REMI