In South East Asia, Vietnam is the only nation that has a rail link with China and basically adopts the meter-gauge trucks except a dual-gauge truck between Hanoi and a border area.
So, Laos had better adopt the dual-gauge between Vientiane and a border area. In the other areas, the country will adopt the meter-gauge trucks.
And, China's blueprint of constructing railways in South East Asia looks like that of Empire Japan in WW?.