Vector graphics are purchased on an as-needed basis. For generic stock, one doesn’t have to go to far to find dozens of options all at pennies to the designer. A pretty saturated market if you ask me.
If I were going to do stock art of any kind for money, I’d find a site that offered me the most bang for the buck, as in the highest percentage offered. It still isn’t going to be much considering most generic vector stock is purchasable for under $25 of which the designer might see $5, a lot of times for a collection of pieces on one “sheet.” Plus there are more and more Free stock sites out there. A little more risky for the user, but nonetheless, a viable competition.
Once you get beyond generic stock art, you run into having to think how and when a piece might be used. You might want to research current news trends, or take it one step further and look beyond what reporters might discover to talk about next and jump the market. That would take far much more work than you’d ever make back in kickbacks from the stock art site.
For custom vectors, we commission those. They aren’t purchased from stock. We hire based on style usually through an agency that represents several illustrators with differing styles and creation methods. Plus we have a few illustrators on staff…but not everyone does.