Not sure whether issue of resignation acceptance letter is in vogue in your Company. It is company specific and many companies follow direct relieving letter after resignation letter. However when there is a specific need for RAC from your end, you should have persuaded your Reporting Authority for the same and should have followed up with him. Always based on human relationship either Reporting Authority or HR will not mind deviate from the laid down process well within their boundary.
Also always relieving date can be requested in the resignation letter and cannot be demanded as a matter of right as based on your role and responsibility your superior might have expected some additional time frame from your end.
I don't think it will be fruitful for you, even if you take it legally and there will not be any legal validity on this case.