ERISA - Pension Issue

I worked as an LPN at a hospital in St. Paul, Mn. from 1976-1984. At that time you had to have 15 years to be vested in the LPN contract to be able to get pension benifits on retirement. Sometime in the 1990's, the LPN association changed the vesting period to 5 years. I have been told ERISA changed pension rules in 1974 and that these LPN pension rules should have been changed much earlier. I would like to get credit for the years I worked as an LPN so I can receive some pension benefits. Do I have a shot. I called and got a copy of the pension rules from 1984 when I last worked at the hospital. Does anyone have any comments.