Is it permissible to take the drug "Xanax" after vowing to God to stop taking it upon recovery, knowing that the病-related symptoms have reappeared, and is there an expiation for this vow?

We ask Allah to grant you healing. Know that seeking medical treatment is permissible and does not contradict reliance on Allah. The Prophet, peace and blessings be upon him, commanded seeking treatment and he himself, along with his companions, did so. Seeking medical treatment is among the permissible means one should employ; it is not a reliance on other than Allah.

Regarding your vow to stop treatment if you are cured: If you said, "I vow to Allah" or "A vow to Allah is upon me," then this is an oath. If you are not cured, then you have not broken your oath. If you are cured and you see that continuing the treatment is better, then you may expiate for your oath and continue the treatment. However, if you did not attribute the vow to Allah, then this is not an oath, and you are not obliged to fulfill it because it contradicts the Sunnah (definition).