Can my Employer ask me what my time off is for when using Vacation time, or if I have to go to the Doctor, what is the Law regarding time off

If it's vacation time, no. If it's sick time, I believe your employer can ask for a note from your Physician.

Employment contract or agreement?

You are entitled to use your vacation time how you wish... it is time earned and due to you.
You are not required to give any explanation as to what you do in that time or where you go.

However if your employer is just being friendly then the choice is yours.

If you have time off for a Doctors appointment, which needs your employers permission as it's in working hours, then obviously you would have to say it is for that appointment, but you would not have to say why you need to see a Doctor, that's private.

If you have already booked some vacation time off for the appointment then that remains your private business.