It seems that a lot of people are confused about the differences between Photoshop's vectors and those in Illustrator. There are differences between Illustrator and Photoshop in the way they approach vectors but that's the only difference. There is no 'quality' difference. You always have to look at the output. Even if the vector tools in Illustrator are more advanced... in the end they will end up as pixels or dots and that's where Photoshop and Illustrator meet.
The fact that there are two programs has two reasons;
- Marketing; Photoshop's vector tools are still kept very simple, forcing many users to buy Illustrator. Lots of users have both Photoshop and Illustrator; count the cash $$$

- Vector images and bitmap images don't really work together in one work area, since bitmaps are based on pixels (resolution) and vectors are based on mathematical formulas.