Need some clarity on cancellation policy

On a property I have listed, we have the moderate cancellation policy in effect. This means that if they cancel at least 30 days in advance they get 100% refund. They are 3 days inside 30 days until they arrive. If the cancel today, they're enrolled to a 50% refund.

My question is what is the 50% refund a percentage of? Is it 50% of the initial deposit (50% due at time of booking) they paid, or 50% of the total booking?

Total booking is $4K. Collected $2K. Balance of $2K is due now, but haven't received it. If they cancel, how much do I owe them?

If they had paid in full at time of booking, I'd owe them $2K.

What about the fees they paid? Does VRBO refund the booking fee and all taxes? I'd think so. In all honesty, in 10 years renting out through them, I've never had anyone bail out this close to their arrival date.

Any help most appreciated.