After six failed attempts, I think I've finally come up with a border that works for my large rectangle:
It's the willow border from the myrtle leaf shawl, also in VLT, which is the one I've been trying to add for three days now. Last night I managed to figure out the tension and joins well enough to make me happy, and I should (according to my admittedly sketchy math abilities) have more than enough yarn to finish. Although not too much more, so I'm keeping my fingers crossed!
The color in the photo isn't very accurate--the yarn (Posh Eva in Sari) is much richer and more beautiful in real life. I'm still learning how to use our fancy new digital camera.
Since the border repeat is so much shorter than the center panel, it's interesting to see how that affects the color changes--it's less stripe-y and there's no pooling. Not yet, anyway.
Here's my progress so far--the color is closer to true in this photo: