By default, the governing law for contracts between a foreign-owned company and other companies in Vietnam is Vietnamese law.
The 2020 Investment Law allows for the parties to choose foreign governing law for contracts relating to "business investment activities" if it is not inconsistent with Vietnamese law.
The 2015 Civil Code and 2005 Commercial Law allow for the parties to choose foreign governing law for contracts with a "foreign element," which includes:
At least one party being a foreign individual or legal entity
The establishment, modification, implementation, or termination of the contract occurring in a foreign country
The subject matter of the contract being located in a foreign country
To introduce a foreign element into a contract, a foreign-owned company could add a foreign investor as a sponsor/guarantor for their obligations.