If 20th passes then the buyer can escalate the case and ebay will refund in full with no requirement to return the vase.

You can accept the return before that time, but if you do you will need to pay the return postage by trackable means and when you receive it back then refund in full. So you would be out of pocket the postage both ways.

If you do accept the return and the buyer is trying on a scam they may not want to return it and then do nothing about actually returning it.

If this happens they have 5-days to send the item back, if they don't then after 30-days the case should by default close in your favour.