I usually wait until early spring to prune back shrubs including vitex. Since it is still relatively small, Delta Blues matures to 8-10’, you can just prune back the tips to tidy them up in the spring. When it is fully mature, and needs to be rejuvenated, then you can cut it back a third. It may take a few years to get to that size. To keep it flowering through the summer, check out this video on deadheading vitex.
Thanks for adding a Delta Blues to your zone 7 garden!