Hello, im also new to vectoring. Heck even photoshop, but it wont hurt to share some ideas right?
Critique: Try using a bigger canvas/image size, vectors are ment to be resizable and yet have good quality. Also, try sharpening some edges more. Also, the darker part of the iris should be over the lighter side. Also, some white of the eye is poking out from the side. Also, there are 2 extra curved lines on Twilight's hair behind her horn. Also, the black eye border side should be thinner.
I am sorry if this is too much, but im just trying to help out on minor details.
The more mistakes learned, the better in the future.
Looking good, her right back hoof is a bit wobbly, same with muzzle below lips and the pointy ends could be...pointier for general aesthetics, but overall very good, especially for third vector. What program are you using, if I may ask?