Can an employer change the terms and conditions of benefits, vacation, or pension after an employee accepts an offer in which those are listed?
In general: If the contract specifies some term (or "duration"), then it can only be modified in accordance with its own terms. When the employment contract expires any future employment term is subject to negotiation, and benefits could certainly be modified at that time. There are exceptions to this in labor law, but those vary by jurisdiction and won't be addressed here.
Generally "vested" benefits like accrued vacation time or pension balances are treated like the property of the employee: They cannot be unilaterally reduced by the employer. One notable exception would be bankruptcy of the employer. I don't know where employee claims rank in Canadian bankruptcy law, but they are one of the debts that might not be paid in full.