The contract is for 76 3MW direct drive turbines to go to the Pen Y Cymoedd wind farm near Port Talbot. It is Siemens largest UK deal for the 3MW.
The project is expected to come online in late-2016.
Vattenfall's director of onshore wind development in the UK, Piers Guy, said: "Vattenfall is pleased to have signed up with Siemens and placed such a significant order for its direct drive turbine.
"In addition, it is important for Vattenfall that we contract with main suppliers that are committed to working with the local supply chain for installation, operations and maintenance and so maximising local economic benefit."
Vattenfall signalled it was going to push ahead with the project last year when it earmarked EUR 4.2 billion up until 2016 on wind energy projects across Europe. Pen Y Cymoedd was one of four projects specified.
As part of the development consent, Vattenfall will invest in a £3 million (EUR 3.5 million) local habitat restoration project and a community fund of up to £55 million over the 25-year life of the project.
The Siemens 3MW turbine was launched in 2010 and is the company's first direct drive machine.