From my view point, the most important element in making a vignette successful is what makes any piece successful: to fully execute the composition in support of the focal point. The difference of course in a vignette is that some aspect of the canvas is left unfinished. I think that Lon's observation about fully integrating the figure with the background is solid. It has far more to do with the manner in which the edges, colors and values of the subject marry up with the those of the background .Simply not painting part of the subject, in and of itself, is not enough to make the vignette work. You have to "set up" areas of the painting to accept those integrated edges and shapes.
I think that the manner and sequence in which edges between the background and subject are executed are likewise important. Regardless of medium, the painter has to decide whether part of the unfinished aspect allows the viewer to see the support itself; whether or not the support is toned; or whether background and subject will be painted simultaneously.
With a medium like pastel, it's important not to lay pigment on any area that will remain unpainted, since some pastel supports don't lend themselves well to covering up unwanted pentimento. If you will eventually apply pigment to the whole surface, then the sequence and timing of what is painted when probably don't matter much. If you are working in oil, you have a greater ability to paint over unwanted marks, as long as no hard edges are allowed to dry, and as long as no brushstrokes remain visible in the early layers or stages.Sharp-edged areas where the strokes stop abruptly tend to look as if the mat board or frame was left too large - it's just as important to size and place the negative vignette spaces as any negative spaces.
In your sketch, Tammy, the boy's legs knees are a compositional problem regardless of which composition you are considering. Cutting the legs off farther from the knees or closer to the knees won't correct the problem. The girl's hand and arm on our left will remain unexplained because there is nothing to suggest that there is a support under her hand. The fact that her hand extends pretty far south of the mass of the figures will pull the eye away from the faces.
I guess I am saying that, for this painting, I don't think you can make a vignette work successfully by relying closely on the photo resource you have chosen. Even in a fully painted canvas, I think that you would have to introduce a foreground object to hide the legs. It would be - at least for me- impossible to invent a different lower body language that would explain the boy's upper body language.
I have done literally hundreds of pastel vignettes - probably because I studied with Harley Brown so much when I was starting to take classes (and I remain grateful to him to this day!) and because in class we always worked from life, and thought through the vignette before we ever made the first stroke. I want to include more vignetting in my oils, and I try to practice as much as possible... I find it much easier to work the composition out my open studio sessions than in a planned commission - it may be the fact that I know at the outset that there will NEVER be enough time to cover the canvas, so I don't get worried about it.
Nonetheless, even if I am working from a photograph I would approach the vignette in a purposeful manner at the outset. I say this only because I've had so many failures by trying to 'save' a faulty reverence with an arbitrary vignette,sigh.
I'm attaching a couple of examples -- Yolanda, a water color in open studio (yep, should have left a little more room on the left - but you can still see the edges are vignetted); and Marcia, an oil from life ( here I made sure to anchor the figure to the top, and to crop below the "V" neckline-) I have posted many other vignette samples in the Open Studio section as well.
Review Harley Brown's work for the most lucious pastel vignettes, and it is pretty tough to beat anything that
Jamie McMahan' paints in vignetted oils; his self portrait is attached below, although I see it actully show up as the center image ...oh, I could go on and on , Greene, Kinstler, Sanden to name some other living artists.