By the way, this answer is coming not from me but our payroll expert Alan McEwan. Here’s his reply to your follow up:
Yes the answer would be very different. In Ontario, employees have no say in the timing of their vacation. There doesn’t also appear to be a requirement for employers to give a minimum amount of notice in writing before the an employee’s vacation is to start.
But the rest of the answer would be the same. If the employee does not return to work, he or she may be subject to discipline for what might amount to job abandonment.