CRA indicates that vacation days of a contract employee in the US can cause his Canadian employer to have a US permanent establishment

Art. V.9(a) of the Canada-US Convention deems a "Canco," which derives more than 50% of its gross active business revenue from services provided by an employee to US businesses during any 12-month period in which that individual is present in the US for more than 183 days, to be deriving those services through a US permanent establishment (so that the resulting profits are not exempt from US income taxation) - or vice versa in the case of a "USCo" providing services to Canadian businesses through an employee who is present in Canada for more that 183 days in a 12-month period.

When asked about the Canco situation, CRA indicated that it considers days that the employee happens to be present in the US for personal reasons, such as vacations, to count for purposes of  this 183-day physical presence test.

Neal Armstrong.  Summary of 29 August 2012 T.I. 2012-0433791E5 under Treaties - Art. 5.