Bird's eye bush, Ochna (Ochna thomasiana).
Bird's Eye Bush, Mickey Mouse plant (Ochna thomasiana) originates in tropical Africa. The Ochna species in Vietnam (Ochna integerrima) is called "Mai" which is the Spring flower of Vietnam. This plant can be grow big as a small tree with the height about 6-10 ft. However, most of Mai plants in Vietnam are grown in pots as bonsai. More pictures of Mai plant at the Mai blossom page.
If you want more information about this flower, just search the plant's Latin name on Google: