According to the VHI rule book the 52 week waiting period is for "Maternity or pregnancy - related conditions". Assuming that pre-eclampsia is a pregnancy related condition, then my opinion would be that VHI wouldn't cover it. However, definitely ring VHI to confirm and see if there are any exceptions.
Should it be the case that VHI won't cover the costs, then ask the hospital and the consultant if your wife can switch back to the public system. Normally, they don't allow this as per the Department of Health's guidance on public/private patients. This is an exceptional case though, so they may be sympathetic to your circumstances.
Sorry I don't have better news, but hopefully it'll work out for you (or maybe someone else will have better news!).