I am not really sure, but assuming the participants are still in the union and there has been no separation from service, I would think they would continue to earn vesting credit in their MEP benefits in the future. Not 100% sure of that.
If they are leaving the union, then they are probably term non-vested in the MEP benefits. The MEP would have no obligation to then, nor would they generally care about people who leave the union.
This is something the labor lawyer should be asking about as they negotiate the withdrawal. The "recompense" would have to come from the employer, not the MEP. The employer is choosing to leave so they are the one who needs to "make it right" for their employees if they want them to go along with the withdrawal.