They plan to reach their destination by June 22.
Kim McVinnie, 44, signed up to cycle the 700 km from the north to the south of Vietnam after her 38 year old Auckland-based sister Sally Hewlett, who works at ChildFund New Zealand, twisted her arm and had to pull out.
On average they have been riding about 80km a day, with a day's rest in between and using some public transport to help them reach their destination.
The money raised from the bike ride will go to ChildFund's Vietnam Appeal to reduce malnutrition among children by feeding them a mix of fortified nutrients called "sprinkles". The sisters have also been sampling some to help them complete their challenge.