I'm not a judge, but I am a coach - vault is my least favourite apparatus to coach because there are so many things going on.
There are 4 phases - Run up, First flight, repulsion, 2nd flight, landing
Judges don't tend to judge the run up itself, but if it goes wrong, there is a negative reaction to the other phases.
The first flight is from the board to the table. It obviously depends on which vault is being performed as to what the judges are looking for, but lets take a 'simple' handspring for example
poor technique includes:
Repulsion Shoulder angle
Second flight Legs separated
Failure to maintain stretched body position
lack of height
lack of distance
Movements to maintain balance
Deviation from a straight direction
A judge can take up to 0.5 off for each of these faults and a whole mark if the gymnast falls. So there is a lot of room for losing marks on vault.
As for whether 8.7 is bad, it really does depend on the start value of the vault!
As marie83 said, there really is more to vault than meets the eye! But as to the 8.7, I think that it is a great score! And as you keep working on it, it will get better and better! But don't be frustrated if your score fluctuates, judging is very subjective. Good luck!
Here are the most common things I deduct for on vault:
-Bent legs or piking at the hips at any phase during the vault (and if you do it in pre and post flight you get deducted twice)
-Steps on landing
-Chest down on landing
-Bent arms while blocking off the horse
-Incomplete twists or twisting too early (i.e. doing a 1/4 turn on the horse on a handspring full vault). Twists must be exact.
-Height and distance. This is a big one that most gymnasts don't realize, but it's how I distinguish a good vault from a great one. In the high school rules, I can deduct up to 0.8 for height and distance and I take something on nearly every vault.