A warning from the Vodafone Roaming Terms for the older AnyNet and Anytime sims:
"Vodafone cannot guarantee access to or coverage within any foreign country. If you are unable to receive access to a Vodafone Preferred Network, you may obtain access to another foreign network operator in that country. If so, you will be charged accordingly."
Source:
UK - About Vodafone UK - Legal Information - Terms and conditions - Pay as you go price plans
Be careful to register onto the correct network!