Under Vietnam Law, if you wish to become the Legal Representative of an enterprise in Vietnam, you must reside in Vietnam.
If you leave Vietnam for 30 days, you must grant a power of attorney to other person and it must be noticed to the competent authority.
Upon registration of you as the legal representative of the company, competent authority shall require the company to show documents proving that you are residing in Vietnam at that time. A temporary residence registration issued by police of ward where you are residing in Vietnam shall be sufficient.
During the operation of the company, the competent authority may check whether you satisfy the above requirement or not.