Hundreds of workers protested on Friday over the ending of 46 temporary contracts.
Vauxhall said it was "extremely disappointed" at the demonstration, saying the employees' contracts were always due to run out on Friday.
It said its parent firm General Motors could not afford to lose production of the new car.
Members of the Transport and General Workers' Union at the plant will vote next week for industrial action.
Since the temporary workers involved had been at the factory for two and a half years the union said it viewed these as compulsory redundancies.
Claims of unfair dismissal will be taken to an employment tribunal.