For vignette, the best way to center the effect is to get a centered selection before entering the effect. And centering a selection is pretty easy depending on the method.
Method 1: Custom Selection - this is a button found on the Tool Options Palette when you have the "Selection" tool active. The Custom selection dialog lets you define in pixels the start and end points of a rectangle that represent your selection. So long as the Top and Bottom are the same distance from their respective edges, and left and right are the same distance from their respective edges, the selection will always be centered. Of course this requires that you know ahead of time how much space you want on all four edges.
Method 2: This one is harder but is a lot more flexible. Use the vector rectangle to create your selection. The Vector Rectangle tool requires that you have a fill turned on, and that your stroke width is set to 0 for best accuracy. You can resize the rectangle until you're sure you ahve the size you want. Then use the Objects -> Align -> Center in canvas option to center the rectangle. Finally use the Selection -> From Vector Object command to create a selection from your rectangle. This will be perfectly centered.
Method 3: Use the "Center Layer" script. Make a selection, go to Selectsion -> Edit selection. On the script toolbar select "CenterLayer" and run that script. It'll center the selection. Turn off edit mode Selections -> Edit Selection
Method 4: Use my "Center selection" script - simple script that will take any selection you make, and center it on the canvas. Download it here:
https://drive.google.com/open?id=0B8FlF ... 2NUN1hfZFk
Once you've got your centered selection go to Effects -> Photo Effects -> Vignette.
Turn the Feathered Edge to Zero. Then set the color, opacity, and blur to your liking. It'll use the current raster selection to define the border. Should give you exactly what you're looking for.
