HANOI (Reuters) -U.S. private equity firm Bain Capital has made its first investment in Vietnam, with at least $200 million in the Masan Group conglomerate, and could bring in other investors to raise that to $500 million, the firms said on Monday. The deal is set to boost competition in Vietnam's retail market, as rapid urbanisation draws domestic and foreign investors looking to expand or gain a foothold in the country of 100 million. Proceeds will go to strengthen Masan's financial position, said the fast-moving consumer goods firm, which runs more than 3,200 supermarkets and minimarts in the southeast Asian nation.