The deal is set to be completed in March for an disclosed sum, the Japanese company said in a statement.
Ho Chi Minh City-based Merap, whose products include nasal sprays, eyedrops and antibiotics, has a capital of VND224 billion (US$9.56 million).
Lion is a manufacturer of detergents, soaps, medications, oral hygiene products and others, exports its products to Vietnam mainly through overseas subsidiaries.