underplanting grapes

Once the grape's roots get down deep, they won't interfere with anything.

However. Whatever you try to underplant will likely need feeding and grapes do better without any sort of food. Their sweetness comes from their 'struggle' at taking whatever is in the soil rather than having it heaped on. If you feed your underplantings, then the grapes will leaf out heavily and not produce a lot of grapes. Plus, they'll take up whatever water is available.

Better to simply put weed barrier fabric around the plant and cover with stones.

Logically you'd think feeding grapes, like any other crop, is the way to go, but grapes are the exception to the rule. Think about wine grapes in the fields which are never fed nor watered, and the drier the year, the sweeter the grapes. Other than annual pruning and keeping undergrowth short, they're 'badly mistreated' yet produce wonderful crops.

Mine accidentally got a few shovel fulls of compost and they went rampant with little fruit  That will be corrected this year.