I won't tell you if it's "too late" or not to be a vector designer as I've started uploading vectors last September/October and am just getting started myself.  Who knows if I'm fighting a losing battle or going to reap rewards further down the line?

As for designs currently in demand you can check all of the other micro sites and see what's been downloaded, & what's in the most popular sections. Also keep your eyes open to what's around you.  What kind of graphics do you notice in the magazines that you read? What do you see in marketing for shops?  Browse online and see all sorts of graphics being used.  Use these as things to mark as what might people want.  Also maybe keep an inspiration folder but be careful not to outright copy ideas. 

As for front page content... I don't know about fotolia as much but for most anyplace you need a popular design, great keywords and some luck.    Use the keyword research tool on this site in addition to the keywords you come up with in case you miss a word you never thought of.  Be careful not to spam your images with irrelevant keywords though.

You've gotta just try your designs,  see if they work, and change them up if they don't.   

You'll probably get more advice from illustrators more experienced than me as well.