You bid your slide in the same month you have vacation, as part of your normal line bid. Look up at the top of the PBS bidding page, there is a drop box called "Condition". Click on that and there are several selections, one of which is 'slide vacation' then you put in the number of days (+ means later in month, - means earlier in month) that you want to slide it, you can even put in a range of days, like "10 to 5" days, so if it can't do all 10, it will try to slide it 9, then 8.