In terms of treatment in hospital, your father's Plan B will cover him for treatment as a public or private patient, while his medical card would only cover him as a public patient.
Being a private patient means that, in general, he would have quicker access for hospital treatment. Part of this comes from the fact that his health insurance plan gives him access to more hospitals (i.e. private hospitals).
His medical card will give him free treatment as a public patient but he may encounter waiting times for treatment, depending on what treatment he needs and what hospital he's going to.
Also, things like MRIs, CT scans, PET Scans are fully covered with health insurance, provided you go to an approved centre.
If he's not happy with the price he's paying, he can change to another VHI plan or change insurer. I know that for someone who has (presumably) been with VHI for a considerable period of time, it might seem daunting to switch, but the law allows him to switch to another insurer without starting his waiting periods again and he will pay the same rate as everyone else. Have a look at the Health Insurance Authority's website,
www.hia.ie.