There is really nothing wrong with this vector, no outstanding mistakes, no questionable proportions. In fact, she is rather cute and well done. Her wings are extended, and the way her head and neck are angled gives some insight to her current emotions. You've drawn her in your own, unique MLP style.
But aside from that, what can I say? An artist should put themselves in the shoes of the viewer, and ponder this question.
This vector isn't all too interesting. There is little shading, no highlights. The direction of her eyes has been used, time and time again. There isn't much to ascertain from her expression. The pose is mundane. The angle, well, it has also been used time and time again. Background? Non-existent.
How can you improve this? Well (disregarding the initial description of the prize), first off, the subject will still be the same: This OC.
But you could muck about with the angle, maybe tilt it, make it playful. If full-body isn't necessary, you can omit her legs to focus on her face, especially if her expression is important to this deviation. You can have fun with her pose: flying, hovering, or taking-off. But if energetic movement isn't who she is, give her an item to interact with: a book, a cupcake, even saddlebags. Also, more shading or highlights can be added for depth, especially if you decide to have fun with her pose. You can also make shadows and highlights a slightly different hue from the base coat colour. Maybe some added expression: You can give her a wide smile, or even a game-face: squinting slightly with a small cocky smile. Something to give her personality. A background could be added, even an abstract one that can reflect her personality: stripes, shapes, patterns, play around with the arrangement of colour also. Hell, you can add her cutiemark into the background also.
So, yeah, I've pretty much said all I can. This is the first critique I've written on dA, so I'm hoping this doesn't get you down, but instead give me and you motivation to strive for better (which means a well-spent 2 hours writing this).
I really love the color scheme you've used for this pony, and the shape of the hair! It is very difficult to use brown hair on a pony because it goes out of that vein of brightly saturated and pastel colors and into a more naturalistic place, and it is, of course, quite dark. You've balanced that by using a very muted, light color for her body, much in the way the show tends to pair dark haired ponies with light bodies. The bright reddishness of the hair keeps it from looking drab. I love her hair type... it reminds me of mine! If only you'd shaded and highlighted, as the reviewer said.
That said, the hair is flat inside. I mean, it's not just the shading. It's... flat. There's no fiber inside, no texture. Many artists would use bright or dark brown lines, maybe even black, to show individual hair fibers traveling from root to tip. Some would even shade or shadow those individual fibers or sections. This is especially important in a pony with wavy hair, as it says a lot about her grooming. Are all the waves going in the same direction? Are there "Chunks" that are behaving differently?
Amazing conformity to canon style on the body, though, and the particular deviations your style has in the face is cute.
Sorry for the late reply ;A; But thank you so much! I appreciate this so much ; v ;
About the hair, it's very hard to get what you're talking about in a vector. Vectors are not typically drawn with tablets/hand drawn strokes. They're placed individually. Piece by piece. Stroke by stroke. Vectors take a very long time to make, and it would take a few days to vector each individual strand of hair o/o Also, this was not supposed to be shaded. Just flat color. That's what the prize was.