Is it permissible to consider the additional amount paid to the domestic worker, above her agreed-upon contractual salary, as compensation for her entitled vacation month's salary, especially given that she has not received prior days off?

This increase has no relation to the concluded contract, and it appears it was paid to the maid as a gift or compensation for her day off. It is obligatory to adhere to the terms of the contract, in accordance with the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions," and Al-Qasim bin Muhammad's saying: "I have only found people to be bound by their conditions regarding their wealth and what they have given."