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No, generally a resignation, forced or not, is still a resignation.
Now, if they forced you to resign because you used FMLA and not for any other reason, that is a whole different claim and you could file a discrimination/retaliation claim for that with the US Department of Labor for violating your rights under FMLA as it is unlawful for an employer to retaliate against an employee for using FMLA. That is a separate claim from whether or not you are due your unused vacation time.