It would cost US$67B ... woooow, 16% of total Vietnam GDP! extremely huge project for Vietnam. Just to put this into a perspective if it were in China it would be equivalent to $18T * 16% = $2.9T!
I am not convinced that Vietnam could afford it
16% of GDP is a lot, but it is doable with the right financing deal
if the project is expected to generate consistent profits.
Cost for the Laos Boten–Vientiane railway was $6B or about
37% of Laos' GDP of $16B. Out of the $6B total cost, $2.4B came from equity in the joint venture owner of the railway with China and Laos contributing/owning 70% and 30% of the equity in the joint venture respectively. The remaining $3.6B came from debt taken on by the joint venture with financing provided by the Export-Import Bank of China. This debt will be paid back from the profits generated by the joint venture.
Of the $730M in equity contributed to the joint venture by Laos, $250M was disbursed from the national treasury and $480M was borrowed from the Export-Import Bank of China. So even though the total cost for this economically transformative project was $6B or 37% of Laos' GDP, the government of Laos only needed to come up with $250M or 1.6% of GDP upfront and take on debt of $480M or 3% of GDP.
While China bore almost all of the risk for Boten–Vientiane, the risk was actually limited as the railway is already very profitable since it is the only efficient way for moving freight in landlocked Laos.
Vietnam is probably looking for something similar to the financing structure for Boten–Vientiane, but the problem is dedicated high speed passenger rail simply don't generate large profits like freight rail does.