Welcome to Vodafone Community
See here: Free PAYG SIM
and this link: How can I keep my mobile phone number?
Also a Vodafone store will be able to provide a SIM and start the porting process, your number will be ported over within 24 hours, excluding weekends where numbers do not port on any network.
Also take a look at Voxi still Vodafone. I can't give you a link. Links don't seem to work from a phone.