Oh well, my immediate thought is that I personally don't like the vignetting AT ALL. But that is soooo subjective. It should not bother you in the least.
Other than that I like the way you composed the path to the right of this photo, how it vanishes in the distance - and ... the corn is still so low, it must have been taken earlier in this year, or wasn't it? (Here the harvesters are about all over the place, harvesting the corn!).
I actually like the vignetting, but its a little overdone, which has already been mentioned. But I think it could still be effective using a different method.
The composition is really nice, but 2 things stand out adversely. 1) the sunlight is causing some bright spots and blowout on their clothes, and 2) their heads are blending into the dark background too much.
I think if the lighting was softer, and the subjects were lit a little better, this would be a cracker! Just my 2 cents.
To me I think it would work better if it was more from the couples perspective, like the shot taken 6 paces to the left, so we see the road unfolding in front of them. Plus maybe around evening or early morning. My $.02