Expats working in Vietnam will be required to pay into Vietnam's social insurance program starting in 2018. The social insurance tax currently only applies to Vietnamese employees. The employee contribution of the tax is 8% subject to maximums (about 1,140 USD per month).
Other countries with similar requirements allow expats to cash out their contributions if they leave Vietnam. It's not clear if Vietnam will have this option.
The tax authority is still in the process of drafting regulations to provide more guidance.