Don't know about April but Azamara has a wonderful itinerary Hong Kong to Singapore stopping at both Hanoi and Saigon enroute Singapore. We did almost identical itinerary on Regent Seven Seas Mariner in 2005 and loved it.
Azamara Voyage | Azamara Club Cruises
"Carpe diem quam minimum credula postero"
Seven Seas Explorer - Miami - Venice - Mar 2017
Silverseas Explorer - Arctic - Aug 2017
Seven Seas Voyager - Rome to Beijing - Oct 2017
Seven Seas Mariner - Lima to Buenos Aires - Jan 2018
Silverseas Explorer - Siberia - Jul 2018
Seven Seas Mariner - Australia Circumnavigation - Dec 2018