I'm assuming the following
1) That Congress will pass a bill that includes backpay for furloughed workers. Some might want to debate whether this will happen or not but going by historic precedent and for the purposes of this question, let's assume they will.
2) That the vacation days can be carried over to the following year. I realize that this may not be the case for all i.e. it may be use it or lose it type of vacation days. Since this shutdown occurred so close to the end of the year, it complicates things but again, assume that vacation days do not expire.
Can non-essential furloughed workers "cancel" their vacation and use the holidays later instead or would this be considered illegal/unethical?