1. A vacancy bid is when the airline needs people to fill slots on different airplanes in different bases. For example, Newark based 737 copilot, San Fransisco based 777 Captain, etc. They are required to offer them to current pilots first. If they go unfilled after a certain amount of time, those slots can be offered to new hires.
2. The percentages are seniority. If I am at 80% seniority in my base and seat, i.e. EWR based 737 copilot, that means that 19% of the other EWR based copilots are junior to me. That means better schedule, less likely to be on reserve, etc. There is also company wide seniority, i.e. every pilot in every base.
3. NB = Narrowbody, i.e. 737 and Airbus.
4. Commutability is generally a trip that starts late enough in the day that you can commute to work on your first day of work, and ends early enough that you can commute home on your last day of work. For example, a trip that leaves your base at 4pm, and gets you back to your base however many days later at 8am is commutable on both ends. If your trip starts too early to catch a flight from where you live, and you have to fly in the night before and stay at a hotel isn't commutable at the start. All of this is moot if you live in your base, i.e. you drive to work.
Generally widebody trips leave late in the day, and get back early in the day, which makes them more commutable. Narrowbody trips seem to either start early, or end late, or both, and aren't as commutable.
Hopefully this clears it up a little bit, I'm still trying to learn all this stuff myself.