What should someone do who vowed to wear the legal (Sharia-compliant) dress upon recovery, and she desires to do so but not now? And is it permissible for her to fast three days as expiation for it?

The legal hijab is an obligation that cannot be abandoned or delayed, and its description has been outlined in previous fatwas. If you have vowed to commit to it, then the vow of an obligation is not binding according to the majority of scholars. Therefore, nothing is incumbent upon you by virtue of this vow, but you are obligated to hasten to wear the hijab, and it is not permissible to delay it.