Thank you for your question!
Please inform that a Vietnam visa can be extended 3 times at maximum. The longest Vietnam visa entry is 3 month single/multiple one. That means you can stay continuously in the country without exiting for 9 month. After that you need to exit the country and re-apply for a new visa to get back. Your new visa then can get extended for another 9 months as the same as the former one.
In case you can obtain work permit, thing is more easier. Your visa can be changed into working visa with validity of your work permit.
Should you have any other concerns about Vietnam visa extension, just feel free to contact us at anytime.