This must be a requirement specific to your individual province or industry.  In Ontario, non-paid vacation pay is not part of T4 box 14 so it would not be included.

Under Setup, Settings, Payroll, Incomes is the control for what tax is charged.  WCB/WSIB is not included here so the control must be built in and not user-defined.  It looks like you may have to calculate it manually unless someone else has an option.

As Richard Ridings said, this sounds like a requirement specific to our province.  As an accountant working in BC, I can confirm that Worksafe BC premiums are based on earnings actually paid out in the year, with no reference to any earnings earned but not yet paid.