visa application time limit before entering Vietnam and getting actual Vietnam visa

We received forms for Visa-on-arrival in Vietnam in May 10 with the intention of arriving in Saigon on May 30. On the day before departing US, my wife broke her arm and had to have a cast applied and was advised by her doctor to not fly with the cast. That will delay our departure from the US until June 15. Can we still use the approved application at that time or is there a limit in time after application is made and approved?