What line is this that is capped off?
 

http://filebox.vt.edu/users/justinl/DSC00001.JPG
http://filebox.vt.edu/users/justinl/DSC00002.JPG

I broke one of the nipples on the spider that feeds the oil injectors, and this is capped off. What is it for?

nothing, it was an extra line that i believe was used on Jspec or Espec models.

Is it just a simple vacuum source? I broke one of the plastic nipples on the splitter for the vac lines that go to the oil injectors. Would I be safe using that as a vacuum source for one of the oil injectors?

it goes to nothing and is normally capped at both ends, so no it isn't a good vacuum source. you will need to find a new spider, the injectors do not see a vacuum source anyways since that would work reverse and pull oil into the intake rather than the rotor housing, their air source is pre-throttle plates so it is not in vacuum but just metered free flowing air that goes down and through the injectors into the rotor housings.

Intersting, so air is just pushed through the injectors via the 4 hoses?

the injectors in the rotor housings are actually located in the intake side of the housing so they themselves are in a vacuum so they pull air from before the throttle body.

So since I only have 3 lines to work with, which injector should I leave without a line until I can get a new splitter?

if you have any of the nipples on the backside of the UIM capped off then just unplug and use one of those, they should all be pre vacuum air sources.

you could also split off one of the other lines and Y to 2 injectors or just redo and make your own splitters, i have done it before with a Y from the large line to 2 medium size ones, then off each of those 2 medium size lines i went to 2 small size lines for a total of 4 to all the injectors.

Use the correct nipple for the oil nozzle air bleed. There are three or four nipples (depending on model) on the back of the UIM directly behind the TB, one of which is bigger than the others. That is the correct one.