PPI in Illustrator

Vectors have no resolution until rendered, and then at the resolution of the output device.

As Ton mentioned, PPI only becomes relevant when you are dealing with certain effects, or when placing images.

In the case of effects, the Higher the Raster Settings, the better these will look; If you are going for professional printing, you'll want the best, so 300ppi.

As for images, you need to place them with sufficient PPI and size them accordingly for your output situation. If you place a photo that is 300ppi at 100% in Illustrator, you've got 300ppi for output...enough to work with for quality print. If you, however, size that same photo at 400%, you have conversely reduced the effective resolution to 75ppi, so you need to watch this.

In your Links panel, you can show the Effective Resolution for anything you place.