The high definition for this vector is very good! My pictures (none being vectors) tend to be blurry because I upload them from my phone. I personally have never seen Derpy in this position. Actually, I never have seen or even IMAGINED Derpy crying, so I think this is very original. I've never done a vector before, so I don't know about technique, but it looks nearly identical to the show's graphics. the expression on her face definately shows her emotion, and the description gives it some more character. In total, it was fairly "impact-ful". I was trying to decide between giving you 4 stars or 3 and a half, but I finally decided on giving you 4 stars because if the situtation (in your description) and expression. I think that it would be just a bit more impacting if there was a background to the vector to show what is happening that is so sad, but I can definately see why you didn't make one: There are not many things that describe her supposed removal. I also found that your explanation is stated in the description. You have very nice shading. It is very subtle, so it wasn't distracting. And yet you gave it just enough to almost look 3D.
The only things that I do not particularly like are the low angle of the eyes and that the tail comes from so high on the body. Still, these are not noticable unless you are actually searching for problems.