The app will tell you that your 'vehicle is in use' for two reasons. Either your app is stuck in online mode, or if your vehicle is a shared vehicle, then it could be in use by another driver. Please contact Veyo Support here to have an agent assist you with making your vehicle available.
We encourage drivers to make sure they set their status to 'offline' before you sign out of the app for the day to prevent your vehicle from being stuck in online mode.