I know this question is a few years old now but I was having a similar issue today So I’ll leave this here for future people who may stumble upon this at a later date…
Make sure your render dimensions and percent match your render layer settings used in the compositor before adding the node… I had a scene set to 1920×810, 100%. I set the percent to 25% to run some tests and forgot to switch it back before trying a vignette on the full-res render, so the percent was mismatched… My elipse matte was resizable, yes but there was a weird secondary matt that was clipping it. Then it dawned on me… that little mask is roughly 25% of my shot… Bingo. Deleted the mat, reset the percent to 100, and added a new matte… worked perfectly.
