It's my understanding that vacation buy-back (vbb), when offered by the company, is done in seniority (there's that seniority stuff again) order. So even if they announce vbb for a particular month and a particular seat, they will only buy it back from the most senior guy to the least senior guy until they have met their goals for crew staffing. In effect, most guys who put in for vbb will probably not have their request honored. As well, I can't remember when the company has ever canceled someones vacation, unless for some reason such as it interfering with training. It just doesn't happen very often.
Advance volunteer is done on a first-come, first-served (earlier time stamp) basis, however, you must go by the rules for advance volunteer or the computer will simply kick your request out. You can't put in for AV until 7 days prior to one hour prior to show for the trip you want. In other words, if you're trying to AV for a trip on the 10th of the month, where the show time for the trip is 1000 LBT (as an example), you can't submit for that trip until 0900 LBT, on the 3rd of the month.
So if you really need/want to get that particular trip, the best way still might be to pick it up in a make-up status. A lot of times if you are waiting to pick up a trip AV, and are just monitoring VIPS open time, you'll see it disappear as someone has either trip-traded into it, or picked it up in some other status. That goes double for the really good trips. Personally, I don't sell back vacation, nor do I ever do a make-up trip. Especially for things like MUS (make-up sick). But it's certainly within the contract and as such I have no problem with guys doing that. However, AV is a good thing, both for the company and for the individual pilot. The pilot makes time and a half, and the company gets someone to fly that trip, without having to use a reserve guy on it, thereby giving them additional flexibility. A win-win situation, except it could be argued that by picking up these extra trips, you deprive another pilot from getting hired here. But (IMHO) as has been the norm in this industry, no one really cares about guys not already on the property.