They had briefed officials on Vietnam’s human rights situation ahead of an annual dialogue.
But US silence on human rights has experts looking to other countries to raise the region’s grave problems.
Officials say another Vietnamese died while in captivity.
Brotherhood for Democracy members Le Anh Hung Truong Van Dung decline to turn up, saying they have done nothing wrong.
Activists say Hanoi's arbitrary detention of bloggers and rights defenders violates the spirit of APEC.
Hoang Duc Binh could face 2-7 years in prison following arrest for leading environmental protests.
Forty academics urge the Communist Party to free "Mother Mushroom" and another woman jailed for posting anti-state propaganda.