The fourth of the six Kilo-class submarines that Vietnam has contracted to buy from Russia is scheduled to arrive in Vietnam next month.
The submarine codenamed HQ-185 Da Nang is currently carried by the Dutch-registered cargo ship Rolldock Storm which is on its way to Vietnam.
It is docking at a port in the Canary Islands off the southern coast of Morocco for fuel filling.
The submarine is scheduled to be delivered to Cam Ranh Port in late June.
Meanwhile, the fifth submarine codenamed HQ-186 Khanh Hoa completed a two-week trial run and returned to Svetly Shipyard (Kaliningrad) on Thursday, according to Russian media.
The first three submarines named after Hanoi, Ho Chi Minh City and Hai Phong, arrived in Vietnam in 2014 and early 2015.