My property buyer/s pulls out of the Agreement of Sale and completion of contracts due to my failure in any other way, i.e. failure to provide vacant possession of the property?

You will not be able to trigger ClozeSure.

You will almost certainly be liable to return the deposit to the buyer (who may bring a claim against you for the costs, damages and losses they have incurred due to your breach of contract).  The ClozeSure Guarantee cannot be triggered by a seller who is in breach of their sale contract, which would include failure to give vacant possession on completion of the sale. 

Note that you will not be entitled to a refund of the ClozeSure fee.