You answered your question yourself - neither a T4 nor tax receipt lists your vacation hours or your work hours for that matter. You are worrying too much over small details. The Immigration officer has no time to go find out when you took your vacation and for how long.
All they are looking for is that you were on full-time payroll for 2 years
Well, i'm worrying 'cause we're in the state of global financial crisis and the job can be lost anytime they decide to cut company costs (actually everybody awaits for layoffs to start after the last board of directors). What if I was working for less than 2 years and was then sent to unpaid vacation for several weeks/months instead of being fired? What are formalities here?
CIC wants T4 slips, obviously, there would be 0's in there for this period, if there will be any slips at all. Will this account to building up working experience or not?