With regards to iStock, often Reviewers will suggest looking at the "Needed Files" section - so may be worth considering:

http://www.istockphoto.com/illustrator_7.0_neededfiles.php

My sales at 123RF and Fotolia have always been tiny compared to iStock & Shutterstock, so I tend not to spend too much time worrying about them 

Also on iStock, putting your images in lightboxes with links in the "Photographers description" section of each image must be a good way to help increase sales - the reason I say this is because I find I'm constantly clicking these links, exposing me to tons of images that otherwise I would never see. I haven't had time to figure out how to do this yet, but it's definitely on my agenda!! 

Steppysteph, start your own lightbox on IS, and put others' work on it too, you'll get a lot of exposure.

How to improve vector sales?
Improve your skills, and produce stuff that sells. That's my experience.
You don't need lightboxes, image of the week, free images,Istock hot shots, contests etc. You don't need all that extra exposure and you don't need to be exclusive.
Produce quality and it will cream to the top.

If you don't believe me.... look at the Istock Hot Shots, image of the week, free image of the week and the contest winners. Their sales aren't always that great, and sometime even pathetic.
Istock puts those images there just to show the clients that they have variety. They don't need to give exposure to the bestsellers. Bestsellers find their way to the top on their own.

Actually image of the week on istock (at least for photo) makes a huge difference in sales, if the image is at all stockworthy (speaking from experience). I think the vector image of the week gets a bit less exposure.
A lot of getting good sales on istock for a particular vector image is a combination of research, luck and skill (in roughly that order). Even new comers to istock can get lucky. You have first of all to produce a really saleable image. if you look at the top selling vectors in the last month, the top seller was one of the very first uploads by a new vector artists that just got picked up by buyers a few months ago. It's very simple, there are maybe a couple of hundred almost identical fies on istock. So they just got lucky both in terms of getting the image approved as such simple files tend to get knocked back, and by getting a quick download which bumped it up the pecking order of the best match search. It got a couple more and hasn't looked back since. 
If you upload a lot of good quality saleable files then it means the chances of getting lucky are increased.

(My vectors are more to please me than to generate huge amounts of income - it's a hobby for me, and I don't do the research and don't generate large volumes of simple files. So my sales have been steady but a long way from spectacular, and they are only just starting to get going after submitting vectors for six months)

I added some of my illustrations to lightboxes. Yes, of course it is always producing quality files that will make it sell. I guess I am still new at this and don't know what really sells. Sometimes its the easy files to create that sells. I have less than a month's experience in selling illustrations so hopefully I'll get better at this soon. Thanks!

Hi 9lives, I love your people illustrations! I still can't draw that well so I stay away from people illustrations. My very first attempt at vector drawing is a person illustration. I spent 2 weeks doing it and I'm not satisfied with the result so I don't upload it. Maybe after a few months of practice I could draw decently hahaha!