The web prices will always include the 10% discount (where it's available to the general public), so if the web price and the salary slips price match, then chances are that your wife is getting the maximum discount.
If you want to be doubly certain, look up the plan on the HIA's website. The price on the HIA's website will include the group discount where it's available to the general public. Also, the documentation that VHI sent at the last renewal should set out the premium before and after discounts, tax reliefs, etc.