No, you do not have a case, unless you had a written contract guarantying you that you would vest or guarantying you employment until March. In the absence of a written employment contract, all employment is employment at will; an employer may terminate you at any time, for any reason--and particularly, to avoid having to give 55,000 RSU and a $20,000 bonus to someone who will leave right after getting them, and who has already told the employer that he or she will be leaving. In the future, do not announce your intention to quit or resign until after vesting, if vesting was only a few months off.