Answer: Most often, grapes are planted alone. I believe the reason for this is to allow the grapes the maximum anount of sunshine and soil moisture and fertility so that they can produce better.
You might experiment with a morning glory (or any annual vine you like) or two and see what happens. The grapes might outcompete the morning glories so that the display is poor; it is also possible that the morning glories would twine themselves tightly around the grape and constrict it in some way. It is also possible it would work out fine, in truth I have never seen this done.
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