US firm Rosen Partner is planning to build a themepark in Hanoi, The Investor is reporting. There is a huge dearth of additional information in that the article mentions no other details at all. Rosen Partners is actually a real estate firm and appears not to have any amusement park interests on its books.
Update: That might not be correct. According to their website they may have a stake in the Nine Dragons Waterpark in Zhejiang, China.