The provinces of Quang Binh and Ha Tinh registered 19 deaths in floods, and seven other people were killed elsewhere in the central region.
At least 8,400 hectares of crops are under water in Ha Tinh and some 27,000 houses have been damaged.
The province said it urgently needed "at least 60 tonnes of instant noodles, 8,000 boxes of mineral water and 1,000 kilogrammes of chemicals to clean water for domestic use".
Helicopters and hundreds of military personnel have been mobilised to bring in supplies and help evacuate residents.
Several trains between Hanoi and Ho Chi Minh City have been interrupted as rail tracks are under water while thousands of cars and trucks were stuck on the main national Highway 1, which had been damaged.
The fireworks explosion killed three foreigners and one Vietnamese person during preparations for Hanoi's showcase millennium celebrations, officials said Wednesday.
The nationalities of the foreigners could not be officially confirmed, although Singapore's foreign ministry said one of the dead may be a Singaporean woman.
VietnamNet online news service quotes the director of the Hanoi police, as saying four people were killed when two containers of fireworks "burned" and three other people were injured.
"The reason for the fire is due to mistakes in the transportation process," he said.
Witnesses said they heard the blast before noon near Hanoi's My Dinh Stadium, where the city will celebrate its 1,000th anniversary on Sunday.
The explosion sent metal debris flying and windows at the Vietnam Sport Hospital and apartment blocks about 200 metres from the blast site were shattered or blown out.