You will have your Vietnam visa extended in case your visa expires. For eg., your  visa is a one month entry visa, but you would like to stay in Vietnam for 45 days in total, you need to get your visa extended 1 month more. Mostly the visa could be extended once. It would be best for visa applicants to have visa extended about 10 days before its expiration. The easiest way is to ask an agent do it for you. Click here for procedures of visa extension.