"Assuming PBS mimics the current VTO process?"...Who said that?
Comparing a VTO (Vacation, Training, Other) line with a Preferential Bidding System is like comparing a lightning bug with lightning... In one, you are picking up trips from people who are off or training (10 to 15% of a given crew force each month), in the other you are starting with seniority number one and BIDDING for your line of flying through individual trips.
Imagine if you're number 200 in a given seat with a total of 300 pilots. You have vacation the 10th through the 16th of the month, AND you're a commuter. After 199 other individuals have their schedules built by the computer, then it comes to your preferences..All the D/H trips have been requested and built into more senior pilots' lines, but there is a trip that ends on the 9th and another that starts on the 17th that still need to be filled. Bam, there's your line. Still need to build in more block time? How about dropping in an out and back on the 20th, followed by three days off, another on the 24th, and then again on the 28th? The computer doesn't care that it can't honor your commuter request, nor the fact that it has built trips on either side of your vacation. Those are just the trips that it has left to choose from that fit your "footprint".
But I digress, my point is that under a PBS system, the lines of flying are built AFTER you've bid for specific trips, days off, etc.. Depending on how the program was set up, they absolutely could "drop in" trips on either side of your vacation week. Some people obviously would like the flexibility afforded under the system, but at what cost? PBS could potentially create monthly schedules just as onerous as the daily ones created by our efficient friend "the Optimizer".