Employment Law Questions? Ask an Employment Lawyer.
In Virginia, the employer would owe your accrued vacation at termination only if they have a company policy stating that they pay out vacation at termination. If they do not, then the law doesn't require them to pay it out. Even if they have a policy stating that they must pay it out, they don't have to do so if they also have a policy which excludes payment if you don't give notice.
Now, you appear to be asking if they can refuse to pay out the vacation time that you were already on and approved for. If you were on approved vacation, they can't do so unless their vacation policy specifically states that you have to come back to work following vacation for it to be paid out. Otherwise, you'd be able to sue them for breach of contract concerning vacation.
As for back dating your termination/resignation date, they can't legally do that. It would be a violation of insurance laws and a breach of contract. Your insurance can't be terminated until a qualifying event happens. Your resignation wasn't until after the surgery and they did NOT terminate you earlier, so they can't change that reality.