Hey,
The visa vacances travail act as a "titre de sejour" and is one of the visa that don't need to go through the approval of Direccte/ofii, so your employer just need to do a DPAE (pre approval job form) and send it to the URSSAF.
https://travail-emploi.gouv.fr/IMG/p...a_14738_01.pdf
They will ask you for your "N° Sécurité sociale" since you probably don't have one. They can put "premier emploi" ("first job") or create a fictive one just to identify you.
That form needs to be sent 8 days prior to your job starts.
You can show him this document. (page 5)
https://circulaires.legifrance.gouv..../cir_39718.pdf
"2• le VLS-T permet il son titulaire d'entrer, de séjourner et de travailler à titreaccessoire dès son arrivée sur le territoire français."
Means you can work day 1 when you are in France without any other documents.