First of all, no one says butter extract anymore, because it's like totally fraudulent. The government requires an extract to include at least a few molecules of the original thing, and butter extract never had anything in common with butter.

Imitation Butter Flavor, now, that's a different thing. Imitation Butter Flavor is a mixture of water, propylene glycol, artificial flavor, and FD&C yellow 5. Who wouldn’t want those ingredients in their Mother's Day cake? Considering all the red food coloring you'll be mixing into the batter, though, are a few more artificial ingredients going to matter?

We would not try to find a substitute for Imitation Butter Flavor. You can substitute vanilla, but we would probably just find a different recipe. Hey! Here's one — a Red Velvet Cake recipe that uses actual butter, not Imitation Butter Flavor (and also not vegetable shortening, which we find to be a fair bit more troubling than butter).