The vendor seems to have done a great deal for himself, he has agreed a purchase price with you and now he's renting out the house, so winning both ways, keeps getting rent and gets your money whenever he gets the tenants out.
Ask your solicitor about the completion date, has it passed? could your solicitor threaten the vendor with non performance of contract? a threat could be all that's needed, there is no shortage of available rented accommodation, i cant believe anyone would need more than a week or two to find somewhere.