The Viking invisible zipperfoot undoubtably makes stitching easier, but I think what really messed me up the first time was not ironing out the teeth well enough to sew up close to them.
I think that Christine Haynes' tutorial on invisible zippers in the Emery Sew-Along series of blogs actually scared me from ironing it enough. I'm not sure why she says what she says because it sounds very serious and bad to overpress... but I think underpressing yeilded a really awful result.
"Carefully take your iron and press the teeth away from the zipper tape. But be careful not to over press it! The key is to get the teeth of the zipper standing straight up so the tape and teeth form a 90-degree angle. Do not press the teeth flat!"
So, I looked at numerous other tutorials and the instructions on several invisible zippers of mine from various decades of vintage to fresh from JoAnn... and they all encourage one to press flat. So I did. And as I was sewing the zipper through the perfectly Viking-sized groove in the foot, it just tipped itself into the perfect 90 degree angle that Christine Haynes described in her blog.
So either I way-overthought what she had to say, or she was cautioning people who were way more aggressive ironers than I am.
Whatever the misunderstanding, I am very glad to have this sorted out. Also, I am excited for this bit of vintage barkcloth that I've had for years to become a dirndl skirt once I get over the excitement of installing a properly invisible invisible zipper.
Thank the sewing gods for that! Now I can dream of many more Emerys and Cambies and other beautiful dresses with the zipper installations of my choosing, not limited by ineptitude to only one way of doing things.