I belong to Union 2150 and with the company I work for I do not get paid vacation. Any non-union full-time employee in this company is eligible for paid vacation. How do I find out why being a union member, I do not get the same vacation time and pay?

First place I'd look is the bargaining agreement negotiated by your union.

You might also want to speak to your union representative and tell them that you don't like this part of the agreement; if enough people do that, they may take it into account next time they are negotiating.

They may also be able to explain the reasons behind the agreement.