Here's a basic guide showing the common issues in Vectoring MLP Characters.
Here as a list again: Fills not reaching or going outside their outlines; Ears having pointy tips (should be rounded, the only exceptions are Luna, Soarin and Mrs. Cake); Bad colours (in most cases, you can check them on our colour guide, which is linked on the left side of the group's main page: mlp-vectorclub.minus.com/mbiv9… ) Lack of proper tapers; Corners being cut off due to a low miter limit; Stroke width being inconsistent throughout the individual parts of the vector (the only exception is when the artist attempted to create a feeling of perspective); Outlines being too thin or too thick in the vector overall; Strokes being wobbly; Missing nostrils, eye highlights (two stripes of colour on the iris) or eye reflections (white circles); Legs in the background lacking a darker fill; Layering issues (objects being incorrectly layered under or over something), includes eyelashes from the far eye in the 3/4 view being layered under the head stroke; Inclusion of raster elements (all submissions must be 100% vector); Lack of downloadable source (vector) files (only necessary for show traces); PNG image not being downloadable or having a resolution smaller than 3k pixels wide or tall; Exporting image with a white background. Using Program Tracing to convert image to Vector.
Update: 14/04/13 - Added Soarin to the exception on having pointy ears. Update: 29/08/13 - Added Mrs. Cake to the exception on having pointy ears.
If you made it in a vector program (Inkscape/Ponyscape/Illustrator), and did not apply a background color/object, they generally export the images with transparent background. In Inkscape/Ponyscape, they make any non-filled areas transparent on default based on the export area. In Illustrator, you have to set the option of transparent background in the export window. Make sure to remove/hide any background colors/shapes/layers you do not want.
If you're just importing images that have a background color into these vector programs, you cannot make the background transparent (unless you make a tedious process of cutting/clipping). Though using said images defeats the purpose of vector formatting/programs since an image, in and of itself, is not a vector but a raster.
It depends on how you want it. In general, you will need a Vector Graphics Program like Adobe Illustrator or Inkscape. They allow you to save in native vector files (AI and SVG) as well as export your vector art in general image files like PNG and JPEG. Do remember that these image files are not vectors, but exported images of Vectors. Simple guide on the differences fav.me/d5ceywa
For Deviantart submissions, I create a zip file/folder with the vector file - AI (Adobe Illustrator files) or SVGs (Scale Vector Graphic files) inside the folder. When uploading the zip file, DA will ask for a preview image. Make sure to allow download. Once everything is done and submitted.
Hey look someone that realizes that a png is a raster and not a vector image. I get so upset when I'm looking for things and it says VECTOR but the download is a png and it's like no. Then people ask me if I do vector requests and it's like no all mine are PNG's, they aren't vectors. I'm working on doing them fully vector but I usually only do my lines in AI.
Actually ponies ears have rounded corners! This makes sense because the toys have this standard and sometimes in the series when you get screenshots or in official merchandise box art there are some very noticeable mistakes. This might because they're working under pressure and may have fewer resources to work with.
Line variation on the body, especially face and ears leads to an asymmetrical appearance that is just subtle enough to be creepy. This leads to a Derpy. with things that aren't naturally symmetrical or would be moving like a mane, tail, or background objects it can work to make it look more appealing.
Traces (of images you've made for the purpose) are not bad if you know what you are doing. You should heavily Simplify them, usually, and then do a little cleanup. In Inkscape this is Ctrl+L, you also have the option to simplify during tracing by increasing 'Optimize paths' tolerance (and 'smooth corners' threshold, but this takes more experience to set intelligently since it's basically a 'curviness' control.); The 'Smooth' option in the main tab is not recommended even though it simplifies your curves, because it is based on a gaussian blur, meaning the edges of EVERY feature are rounded
For MLP style you probably want to avoid making too high-res base images, which produce spurious 'details' that then have to be simplified away. 'Large pixel art' is a nice scale for your base image if it's clean B/W (no greys/AA), to produce sharp edges without excessive detail.
But you shouldn't trace a color image (eg a cutout from a screencap), usually, it's too uncontrollable and unoptimized. Only trace outlines that you have made explicitly for the purpose. Controlled trace + cleanup can be much faster than manually vectoring everything.
also, the most common issues that i see in vectors are the strokes too thick (mostly in vector comics) and bad colors. but nothing's perfect, i presume. at least, this is remains a good representation regrouping the common issues that a vector artist mays encounter in his life