Japan's biggest bank will buy Merrill Lynch's 49 percent stake in Mitsubishi UFJ Merrill Lynch PB Securities Co, the private banking venture formed in 2006, for an undisclosed sum, the companies said in a statement.

A source with direct knowledge of the matter said the value of the deal is about 40 billion yen ($482.3 million).

The fate of the joint venture has been speculated upon ever since MUFG invested in Wall Street rival Morgan Stanley (MS.N) in 2008, taking a 22 percent stake in the firm.

The joint venture had operating revenue of 25 billion yen and net profit of 6.8 billion yen for the year ended in March, 2012. The value of its deposited assets stood at 1.8 trillion yen as of that date, the statement said.

Separately, another source said MUFG plans to buy 20 percent of VietinBank CTG.HM from the Vietnamese government for about 60 billion yen ($720 million), as Japan's cash-rich big banks expand into fast-growing Southeast Asian markets.

(Reporting by Shinichi Saoshiro and Taiga Uranaka; Editing by Michael Watson and Muralikumar Anantharaman)

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