Prime Minister Pham Minh Chinh also sent condolences to his Libyan counterpart Abdul Hamid Dbeibeh.
The same day, Minister of Foreign Affairs Bui Thanh Son also conveyed sympathy with acting Minister of Foreign Affairs and International Cooperation Fathallah al-Zani.
A powerful storm brought a torrent of water that bursted dams above Derna on Sunday night.
Hichem Abu Chkiouat, minister of civil aviation in the administration that runs eastern Libya, told Reuters more than 5,300 dead had been counted so far.
Derna Mayor Abdulmenam al-Ghaithi told Saudi-owned Al Arabiya television the estimated number of deaths in the city could reach 18,000 to 20,000 based on the number of districts destroyed by the flood.