Can my employer deny me vacation accural,for wrongfully changing my contract to a lesser scale 25 years ago. It was just discovered last month that this had happened.

I work for a union out of washing DC, I was laid off in January of 1994, but worked for the company through out the year while being laid off. the company when they brought me back, rehired me and put me on the contract that paid less money and started my vacation time over again like a new hire. I recently discovered that the contract then states that I have to be away from the company for 6 months for vacation accural to sease. I was never away from the company for more than 3 months at the most and worked around 8 moths out of that year. I was brought back in October of 1994 and have been there every since. Am I entitled to back vacation pay and lost wages.