Combine in a large bowl the broccoli, cauliflower, mushrooms and green pepper. Whisk together in a small bowl the corn syrup, oil, vinegar, onion, poppy seeds and salt.
Pour corn syrup mixture over vegetables; toss well to coat. Cover and refrigerate at least 4 hours or up to 2 days. Stir occasionally. (May also be marinated in freezer bag.)
To serve, drain marinade.