First for the positive.
1: You've got her colors all nice, and smooth
2: And her lines, are also all nice, and smooth
And Now for the negative.
1: Twilight's cutiemark, looks a little off to me, it seems to be missing a few stars.
2: And Twilight's eyes, while good, also seems to be a little off.
Overall, this is a BEAUTIFUL vector, you've did a great job
You should be proud of your vector, and I'll surely add it to my Twilight Sparkle favorites
Actually, if it's okay with you, can I use this vector?
I'll even credit you