Whisk together vinegar, chopped shiso, and oil in a medium bowl. With a knife or a mandoline, slice fennel very thin.
Working with one grapefruit at a time, slice off ends, then set on an end and use a small, sharp knife to cut off peel from top to bottom, following curve of fruit. Hold fruit over bowl and slice between membranes to release segments into bowl.
Add fennel and avocados along with a few grinds of pepper. Gently toss to coat and season to taste with salt.
Transfer to a shallow bowl.
Cut shiso leaves into thin strips and sprinkle on salad.
