Looking at the HIA's comparison of the two plans, One Plus gives better cover in private hospitals and maternity costs. The out-patient on One Plus is virtually non-existent, but given you have a €300 excess per person on Plan A, chances are you never got any out-patient from Plan A anyway.
The other reduction in cover is that Plan A covers up to 180 days treatment for psychiatric illnesses, where as One Plus covers 100. Even then the 100 days cover is probably enough (I think the average length of stay for mental illness is 40 days per year), but if you or a family member has had treatment for mental illness in the past, it's worth knowing.
In terms of the cover in private hospitals, there is a two year upgrade waiting period of two years on conditions you already have. In other words, the extra private hospital cover won't kick in for these conditions, but you'll still have the Plan A cover.
The HIA's comparison of the two plans is here - [broken link removed].
One other thing to consider is that I presume the kids free offer is a one year only deal. So come your renewal, you may be charged for them. Check this with VHI though, as I may be wrong.
If you're looking to save money and want a similar level of cover, the HIA's website lists 11 other possible alternatives here - [broken link removed]. It may take a little time, but go through the various comparisons there to see if anything else suits your needs.
(If the link directly above doesn't work, go here - [broken link removed] and enter the insurer and plan name at the top)