Welcome to Vodafone Community
If there are no log in problems with your online account, you shouldn't have any problems with the application.
I would recommend going into the Applications on your phone, My Vodafone, storage, clear cache and data. Follow this with a reboot and try logging in again. As Vodafone need to locate your account mobile data is needed for the first access and log in.
If problems persists and Customer Services are unable to help, follow this link to speak to the Social Team: Contact Us