Sprinkle cucumber slices with salt in a colander. Let stand until cucumbers are drained, about 30 minutes. Transfer to a large bowl and toss with tomato and onion.
Whisk vinegar, grapeseed oil, honey, sour cream, celery seed, basil, garlic powder, oregano, dill weed, sugar, and cayenne pepper in a small bowl; stir into vegetable mixture. Cover and refrigerate for at least 1 hour to overnight.