The agreement was reached at a bilateral meeting held in Hanoi on Tuesday between Laos' Ministry of Public Works and Transport and Vietnam's Ministry of Transport, the Voice of Vietnam (VOV) reported, citing the Vietnam News Agency.
Lao Minister of Public Works and Transport Dr Bounchanh Sinthavong led the Lao delegation attending the meeting.
The ministries agreed to seek investment and support, firstly to carry out a feasibility study for the expressway.
A senior official at the Lao ministry's Road and Bridge Department told Vientiane Times yesterday that a primary study had already been carried out.
Citing the primary study, Vietnam's Thanh Nien News Agency reported previously that the cost of the six-lane 707-kilometre route was estimated at more than US$4.5 billion.