How to incorporate grape flavor in a chiffon cake?
Grape juice, purchased as such, is often diluted with water and sugar and other juices. Additionally, many varieties of grapes, especially table grapes, don't have very much flavor at all.

Using grape juice concentrate will add a ton of sugar which you might be able to compensate for in the recipe. Unfortunately, in a chiffon cake, the sugar plays a role in stabilizing the egg foam.

Fresh, undiluted, unsweetened, Concord grape juice has a lot of flavor and color and is not overly sweet. I don't know where one can acquire it as I get it from neighbors who grow it. Perhaps homebrew supply store might have some for small wine batches.

Failing that, grape extract could most easily be used, as renisis commented above, but the flavor will be less fruity and simpler- like grape candy.