Go anyway, the vacation funds are yours, not the company's, they have to pay you for it.
You could run into problems with going on an unapproved vacation, but those issues will not be related to loosing the funds that are yours. If the injury is serious enough that you are off work, they can not do anything to stop payment to you for your vacation time.
Me thinks your supervisor is not very well informed, or is just trying to pressure you back to work. If you are unionized, call your local rep and find out.