Your new policy would be back-dated to 1st Jan 2015 - ie renewal date - regardless of what company you choose.
An example often quoted as follows; Even if you are admitted to hospital for surgery and your operation date just also happens to
be your renewal date, you can change to a new plan with a different company on the day and the new provider will pay the surgery costs. Its just the way health insurance works.
Why do you think your current insurer would not provide cover? Once your wife has served initial waiting times when you both
originally took out health insurance, or if you haven't upgraded in recent years, or there are no waiting times still to be served for
any pre-existing conditions, there shouldn't be a problem. You can check it out with VHI regarding what she is covered for in relation
to this issue and ask what alternative plans are worth considering, if any, at better prices, without compromising on hospital cover etc.
You can also do likewise with other providers before making a decision.
Snowyb