The project, 10 kilometres off England's east coast, will feature 15 V112-3.3MW machines.
The order includes supply, installation and commissioning of the turbines as well as a five-year service agreement.
The existing 90MW wind farm consists of 30 V90-3MW turbines, which Vestas services, and was comissioned in 2005. It is also operated by Vattenfall.
The 15 turbines will be pre-assembled and pre-commissioned onshore at Vestas' facility at the port of Esbjerg, Denmark, before being shipped directly to the Kentish Flats site.
Construction of the extension will begin in mid-2015.