It follows the companies revealed a preliminary arrangement last month.
Vantiv investors will own a bulk 57% of the combined group, while Worldpay financiers hold the other 43%.
” The mix of scale and existence the merger will bring is an interesting action in the development of a really worldwide leader in payments,” stated Worldpay.
Worldpay procedures countless payments a day in shops, online and on smart phones.
It runs worldwide, but with strength in the UK and United States markets.
Vantiv is mostly concentrated in the United States, assisting merchants, banks and cooperative credit union accept card payments, in addition to present cards and online payments.
The combined company’s international and home offices will remain in Cincinnati, Ohio, and London will become its “global head office”.
Vantiv will pay 397p for each share in Worldpay, or ₤ 8bn, plus ₤ 1.3 bn to cover financial obligations.
The combined company will be led by Charles Drucker as executive chairman and joint president.
Reporting to Mr. Drucker will be Philip Jansen as co-chief executive and Stephanie Ferris as primary finance officer.
Extra members of the brand-new executive group will be revealed later.
The UK company’s chairman Sir Michael Rake and his equivalent at Vantiv, Jeffrey Stiefler, will stay on the board as non-executive directors.
The board will include 4 Worldpay and 7 Vantiv directors.