You can customize your device settings and user settings on your Garmin Connect™ account. From your devices widget, select Device then Time Format and you can select 24 hour format.
Do you have Garmin Connect setup on your computer?
Are you selecting the Time Format option within the Garmin Connect account settings? This is different than your actual Garmin device.
I have not heard back from you in a while --
Were you able to change the settings within the Garmin Connect account (not the actual Vivofit)?