I have that plant in my garden I have no idea if it has any interest for bees or butterfly that would make it a reason to keep it in my garden. Looks like a false pepper.
It is pokeweed. The berries are poisonous to humans but the birds relish them. The problem is, when they eat the berries, they drop seed everywhere and soon you will have pokeweed coming up all over your yard. And they are hard to pull up. You have to dig them. I would suggest removing it.
https://www.gardeningknowhow.com/edible/vegetables/pokeweed/controlling-pokeberry-plants.htm
If I put a mixture of vinegar and Dawn on it, and cover it with black plastic will that work? The grapevine stumps are entwined in a large bush. The pokeberry is right next to a sidewalk.
'd be more inclined to use a bush killer herbicide as those two plants you mention are very stubborn. If you cut the grapevine about 4 inches above ground then use an herbicide, it should take care of it. Vinegar will not help. It may burn the top foliage but it won't kill the roots and it may kill nearby vegetation.
Or if you want to try the plastic, clear plastic works best during the hottest part of the year.
https://www.gardeningknowhow.com/garden-how-to/info/use-vinegar-in-gardens.htm
Is it poisonous
Unfortunately, manually controlling growth, or glyphosate are the best ways to control this noxious weed.
This plant is, both, toxic and edible. Unless you are familiar with preparation (which is quite time consuming, if I say so myself) it is best to avoid consuming it.
https://www.gardeningknowhow.com/edible/vegetables/pokeweed/controlling-pokeberry-plants.htm
absorbed by the tree into the fruit
As long as you don't spray the pear tree, the chemicals, such as glyphosate, should not be absorbed into the tree. You can hold a barrier such as cardboard against the tree while you spray.
If you are going to dig it out, be very careful you don't dig into the pear tree roots. The pokeberry taproot is long and fleshy and it's hard to get all of it.
https://www.gardeningknowhow.com/edible/vegetables/pokeweed/controlling-pokeberry-plants.htm