First I think what they are doing to you here is ridiculous and cowardly by the union. This is also an all too common scenario - union's often promise things to get membership...unfortunately they probably didn't clear this with the pension plan board. So let me tell you first that you may have a case of fraud against the union notwithstanding any of the following and you shouldn't let this matter go....seek local counsel for this and pursue a lawsuit if you must. The other thing you need to understand is that the union and the union plan are different entities and federal law holds that the union cannot bind its plan to do something....although in all reality to the average person the entities are essentially acting as one.
Second, their claim of ERISA SOL expiring isn't rock solid because you had conflicting information - you were told it was vested, then told it wasn't - whether that should have clued you in that you had a claim is questionable and an issue of fact for a trial in my opinion. Further, in some jurisdictions you must in fact have filed for benefits for a claim to arise. Under the federal discovery rule, "a plaintiff's cause of action accrues when he discovers, or with due diligence should have discovered, the injury that is the basis of the litigation." Guilbert, 480 F.3d at 149 (citing Union Pacific Railroad Co. v. Beckham, 138 F.3d 325, 330 (8th Cir. 1998)). In the ERISA context, the discovery rule has been construed to mean that a benefit claim accrues upon "a clear repudiation by the plan that is known, or should be known, to the plaintiff — regardless of whether the plaintiff has filed a formal application for benefits." Carey, 201 F.3d at 46-47; see, e.g., Miller v. Fortis Benefits Insurance Co., 475 F.3d 516, 520-21 (3d Cir. 2007); but see Hoover v. Bank of America Corp., 286 F. Supp. 2d 1326, 1333 n.12 (M.D. Fla. 2003) (requiring a formal application for benefits to be denied to trigger the statute of limitations and recognizing the tension between the standard adopted in this circuit and others), aff'd without opinion, Hoover v. Bank of America Corp., 127 Fed. Appx. 470 (11th Cir. Jan. 5, 2005).