Vignetting

[Medium]

Not enough info. (You have a history of this problem, please start working on that.)

What do you mean by "custom" shape? A typical vigniette is round/oval and matches the aspect ratio of the image. Do you want the user to be able to draw a polygon with bezier curves as edges as a "guiding shape" for example? How far do you want to go here?
How variable do the colors need to be? Just black or white fading to fully transparent, or a custom color with arbitrary starting transparency? Center color/transparency also freely pickable?
How do you do it so far? What are the problems? What constitutes "fast enough" for you?
Do you want linear interpolation of colors, some sort of spline interpolation, or maybe the possibility to let the user choose arbitrary "key steps", each defining a color at x% from the center? What interpolation between those?

You see: "Vignette" can mean something really simple, or be fleshed out to become a fully capable custom shape, custom colors gradient sub-system.