hehe there is more to vault than meets the eye!
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:
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!
Hope that helps!