Do Thanh Toan, a tour guide, first saw them early this month, and said he has spotted them several times since late July.
When he saw them most recently on Saturday morning, they were rising from the water, mouths open, and seagulls were flying above, he said.
Whales love to eat krill, small fish, zooplankton, phytoplankton, and algae.
Vietnamese fishermen worship whales as their guardians at sea.