Dear Kelly Mae Sharpe,
Thank you for your question!
Please inform that the longest entry visa issued by Vietnam Immigration Department at present is 3 month single/multiple one. In your case, you are suggested to apply for 3 month multiple entry visa first. After that, if you wish to stay longer, you can apply for Vietnam visa extension without exiting Vietnam or apply a new visa to get back in case you go out of Vietnam.
Should you have any further concerns, just feel free to contact us.