Vignettes serve to focus attention on the center of the image. Most of the times they are just not necessary. Desaturation, as a technique, can also be useful but where texture is strong but a bit of colour helps differentiate parts of the image. The big problem with these images, at least in my mind, is the visual flow. The eye usually starts at the brightest or sharpest part of the image and then follows the lines to explore the rest. Center placement of a subject is static mainly because the eye doesn't then naturally flow from one element to another. The usual guideline of using the rule of thirds partly addresses this issue by putting the subject on the 1/3 line, which naturally allows the eye to travel across the center to the other side.
If you want to improve, think about what aspect of the subject you want us to look at, then use the technical tools at your disposal (depth of field, lighting, focus placement, arrangement of the elements in the frame) to focus our attention on that aspect. The advice given by Lew Thorton (who I quote in my sig) is very appropriate. If you apply those basic three steps, I am confident your image-making will improve tremendously.