Westerman, who had been sole head of the US firm's Asian investment banking division since 2013, will become chairman of investment banking for Europe, the Middle East and Africa, according to the memo seen by Reuters on Tuesday.
Richdale, a fluent Mandarin speaker and who also has much experience in Southeast Asian dealmaking, was previously head of investment banking services in Asia, a position she held since joining Goldman Sachs from Morgan Stanley in March 2013.
Vella has been co-head of the financing group in the region along with Jonathan Penkin since January 2014. Before that he was head of credit capital markets in Asia excluding Japan.
Westerman departs with Goldman Sachs' investment banking division in typically strong form in the Asia Pacific region.
The firm has cornered more than half of the region's 2015 block deals, booking $112 million in estimated fees from firms like Japan's Daiichi Sankyo and Chevron, according to Thomson Reuters/Freeman Consulting data.
A spokeswoman for Goldman Sachs confirmed the contents of the memo.