That would be too soon - you need to KNOW that INZ are going to grant the VOC. You also have to take into consideration the details of your contract with your present employer, so you are going to give them the statutory period of notice before the start-date with the other company. And INZ need to know about all that so they issue the VOC correctly. You must be covered for the OLD job all the while you work out your notice, and then be covered for the NEW job as soon as you start there.