In most cases, vacant home insurance can be renewed just a few days before the lapse date or termination date but we don't recommend this. Although there may be no lapse in coverage it can take a week or two for the client to receive the documentation paperwork like the declarations page for vacant home insurance.
If the client has a mortgage, it is up to the client to always be able to demonstrate to the mortgage company that good insurance exists on the property, vacant homeowners insurance or otherwise. It's within the right of the mortgage company to secure forced placement insurance coverage if they believe the client has been delinquent in their duty to always protect the house with good insurance coverage.