There's only one big difference with "Vector" and "Photoshop" when it comes to line work or images. Illustrator uses a true vector line and Photoshop uses a "Rasterized line" Vectors can be upsized, shrunk, and manipulated all different ways and still keep a clean extra sharp line. In PS the raster image is more coarse and can't be messed with causing loss of sharpness in all parts of the image. They also don't screen well and most shops can't really vectorize a really detailed photoshop image properly, they may as well do it themselves.
There's also a few free ones like http://www.inkscape.org/
which IMO is hard as Illustrator, lol. Corel, Xara and others all have the same user tyle of interface, it's what you feel comfortable with learning.