I have no idea what the yellow is caused by but would also love to know. Someone sent me a whole trash bag full of incarnata vines and some of them had the yellow on the leaves. It's not like a yellowing where the leaf is slowly dying. The foliage seems perfectly healthy, just discolored.
Karyn

You've got an Incense passion vine. The flowers are showy purple like this, right?

Incense has a latent virus that was present in the original plant--ALL Incense vines have it, since they're clones of the original. The virus manifests in the yellowing of the leaves when the plant is stressed. When conditions improve, it will go away. It's not fatal to the plant, and does no real damage other than sapping its vigor some. At first I thought it was a chlorotic iron deficiency (I'm on heavy, alkalai clay soil) before learning better.

"The flowers are showy purple like this, right?"

Exactly right. I'm happy to know it's not fatal. Thanks for the info.