I did that for fun, but now I would like it to be at least correct.
How could I improve it, please ? especially the eye...
Thanks ^^

Well, I'm not sure what anime this is, and without the scan to look at, I can't really tell how "correct" it is.

However, I do have a few suggestions you may or may not want to consider. First, the hair-clip-thing at the very top has a broken line. That shouldn't be too hard to fix ^^

Second, you used a darker shade of the same color to outline the hair, but there seems to be a strand of hair above the ear of the girl on the right that is outlined in black.

Lastly, (and this is just kind of a personal taste thing) you might want to do the same thing for all the lines (have them be a darker shade of the color they're outlining). It doesn't look bad with the black lines, but kind of gives... I dunno... a lot of contrast? Changing them in this way would give a softer feel to the image and personally I think it would look better.

Anyway, you've done a really great job on this! It looks really good - you should make it into a wallpaper! ^^

Small elements in the vector need to be fix and the colour in the sky is nice but I think more should be added to it and change the direction of some of the clouds and the light source seems a bit off.

Thanks for these first comments :)

Keco

Well, I'm not sure what anime this is, and without the scan to look at, I can't really tell how "correct" it is.

Sorry >_< It's from the anime Simoun, and here's the screencap :

Ah, now that I can compare them, you really did a great job vectoring it! I think I see what you mean about the eye though... In the original scan, the bottom half of the eye seems to be light blue, and you have it as white. However, I'm not sure how much that would help it... I'd suggest just zooming in really close and redoing it very carefully. I know that's not much help, but I can't really find any technical problem with it, which makes offering suggestions a little difficult ^^;

If you make any changes, be sure to post it; I'd like to see it!

Yeah, totally a great job on your first vector, wow!
and you sure picked a complicated one to! (lol, go all out, right?)
about the eye, I would get rid of that base white color, it looks odd, like she's got a glass eye or something.
a lighter or darker blue would go good. and you don't have to trace the eye perfectly, it's *your* vector after all, as long as it looks good, right? you might consider doing the bottom highlight of color style, like so many other animes have, and just cutting out a little circle for her pupil...
don't really have any scans handy right now to show you what I mean, but you know what I"m talking about, right?
other than that, i would fix the hairclip as others have mentioned, and although I don't feel like it would stand in the way of the submission or anything, it's kind of odd to see different kinds of outlines, you've got black outlines on everything but the hair, so it doesn't really seem to fit together well, but that's just my personal tastes, you don't have to listen to that. ;)