I have grape vines It is spaced 12 feet. Also believe it or not I have two Grape Vines I planted each in 20" pot and I made a wire house for each like the Tomato wire house. I have not gotten any fruit from it yet but yesterday I was spraying it and saw lots of little Grape clusters.

I am not sure what the above post has to do with your crape myrtle question, but I will give you my opinion.

I would plant them 8-10' apart.

The point about the grapes would be that shrubs or climbers planted close together will still develop adequately. Shrubs planted to form a drift, as in a mixed border will often be quite close together. As with a row of shrubs planted to form a hedge, the grouping becomes the unit rather than the individual shrub - it does not matter that their individual tops do not grow as wide as if they were planted as widely spaced, solitary specimens.

"Growth: The plant is a vigorous upright grower when young. The selected plant reached a height of seven feet in three years in the field with minimum care. Height with age may reach 20 feet or more. Vegetative growth is rapid in spring and continues until summer when flowering begins. Only modest growth occurs after flowering begins. Unlike most crape myrtle varieties which produce many vertical stems with moderate side branches, this new crape myrtle produces a vigorous growing central stem with moderate side branches, thus creating an upright tree form. If the central stem is cut or killed then a proliferation of upright stems result."

Here is a link that might be useful: Crape Myrtle shrub named `Whit II`