In order to access the internet on your smartphone using your data allowance, you must make sure that your APN (Access Point Name) settings are configured to the Vodafone network after you have switched to Vodafone.
The new APN you need to add is 'live.vodafone.com'.
You can find this on most smartphones through:
Settings > Mobile Networks or Wireless & Networks.
For guides to using the internet on your handset check out our interactive manuals.