Nice vector! It looks good for a first attempt. I like how you used a clean lineart to do this picture.
Alright, ready for some critique now? To start it off, her forelegs are too straight, they sorta curve some more. Talking about the legs, they are also pretty short compared to the body, you should lengthen them a wee bit, or make the body a bit thinner. Now, to her horn. You should have put it on her forehead, but in the picture it is smack on top - but you can fix that problem easily. However, a problem hard to fix would be her mane. Her mane is made up of beautiful curls. Some artists say it is hard to draw, but practice makes perfect. Her eyes are to round, when rarity's eyes are suppose to be almond-shaped. To finish things off, I also want to point out the ear shape. It looks kind of sharp and unnatural and appears to branch out of her skull. It should be relaxed and nestle cuddly next to her mane. And that's it! That's all I have to say about this. Hope this critique helped! Oh, and here's a reference picture to help you with drawing rarity: [link]