Can federal holidays be part of vacation benefits?

I'm not sure if I understand your question. If you're asking whether an employment contract which provides that the employee will get 15 days of vacation, including 7 national holidays, would be enforceable I see  no reason why not.  However, that doesn't mean that you have to agree to it. If you're asking whether a contract providing that an employee will get 15 days of vacation, saying nothing about national holidays, will be construed to mean 15 days of vacation including national holidays, that would depend on the individual facts of each case.  For example, if the employee had been working at the same place for 10 years and that had always been the vacation policy, I'd say yes.  In general, however, I think it likely that a Court would take a contract like that to mean 15 days of vacation not including national holidays.