As sph says, I don't think uneven fletching makes much difference...at least within the 20 yard or so distance.
When I bought my fletching tool, I didn't realize they had a crossbow nock fixture. So while I was experimenting with modifying the one I had to work...I was pealing off more than a few vanes because they were "not" 120 degrees apart!

Hmmm....I wonder if this really matters?
So I made a few up from a little off to way off the "ideal" 120 degree spread.
My findings....within my little 18 to 20 yard range...it doesn't matter at all. While my experiments were not with the same exact single arrow...it was with the same arrow material, same field tip and the same new Bohning Blazer vanes. Some old Easton 2219, gold anodized arrow shafts...don't even remember what bow I was originally shooting them with.
The fixture I used is a right hand helical.

I do think each vane needs to be as parallel to the other as possible, other wise, the air will be directed one way on one vane, another way on the other vane and possibly "another" direction on the third vane.
This 'will' throw the arrow off tragectory.
How much...depends on how far off each vane is how far you are shooting, and the air density you are shooting at...that is "sea level" or Colorado, a humid day or a very dry day.

If you are looking for consistancy and accuracy...your best bet...save your money and get a fletching jig.

Mike