If you are switching from another serivce prover (e.g eircom) and staying in the same property then you can keep your existing telephone number.
If you are switching to Vodafone as part of a house/property relocation then you may get a new phone number if you move into a different phone code area. Your telephone number is allocated to a defined geographic area called a Minimum Numbering Area (MNA). Your MNA is part of a wider STD (Subscriber Trunk Dialling) area which has a defined dialling code.
For example the STD code for Dublin is 01 and there are 3 MNAs within the 01 Dublin area.
If you are moving from one address within your current MNA to another address within the MNA, then it should be possible to move your number, although there may be some exceptions to this for technical reasons.
If, however, you are moving from Dublin to Cork, then you will be moving from the 01 area (Dublin) to the 021 area (Cork), it will therefore not be possible to move your number to a different STD dialling code area.