Is it mandatory for a representative office to have a tax code? Is representative office in charge of submitting reports to Tax Bureau?

ABC Enterprise which head office is located in Australia, opens representative office in Vietnam. ABC Enterprise is the foreign contractor who signs a contract of project supports and supervision with Urban Management Authority of XYZ in HCMC. In this case, ABC Enterprise is considered to have the legal entity in Vietnam? and Does the representative office of ABC reserve the right to perform tax duties pursuant to regulations?

Ms A works at Representative office  which head office is located in Singapore. In 2009, Ms A took maternity leave in a period of 4 months and got allowance from social insurance accordingly. Is the allowance subject to PIT?

Mr X (obtained a tax code) is the chief representative of ABC representative office which is currently performing Tax deduction, return and payment for Mr X. However Mr X wishes to pay  PIT monthly himself instead of by his representative office. Is the Tax returns, payment generally accepted ?