You are allowed to transfer at Schiphol if you are fully vaccinated and you are able to show valid proof of vaccination. If you stay at the airport during your transfer, you will not have to do any extra tests. However, do check what the coronavirus rules are in the country you are travelling to. You will need to have a recent negative COVID test result with you if the country you are travelling to requires it. Get in touch with your airline to find out if you need a negative test result for your destination.