unless they've gone on holidays to the moon, surely the estate agent has some means of contacting them. Alot of people arrange a holiday around selling a property so the property is unoccupied and clean for viewings, but you would imagine the estate agent would have a contact mobile or the vendors would be in contact to see how things are going.
frustrating I know, but you will probably have to grin and bear it, or tell estate agent your retracting your offer until you know the vendors are contactable.