Add vegetables to a freezer bag or large bowl. Set aside.
Whisk all of the Marinade ingredients together in a large bowl or large freezer bag (whatever dish you are going to use to marinate your beef). Remove 2 tablespoons Marinade and add to vegetables. Toss to coat.
Add beef to remaining Marinade and toss to coat. Marinate 4-6 hours in the refrigerator.

Tzatziki
dice cucumber and add it to a fine mesh sieve. Sprinkle with 1/2 teaspoon salt and stir. Let sit at least 15 minutes then press down on cucumbers with the back of a spoon to release extra juices. Set aside.
Add feta and cream cheese to food processor and pulse until cheeses are mixed. Continue to mix 3-4 minutes until cheeses are very smooth. Add remaining Whipped Feta Tzatziki ingredients, including cucumber, and pulse until smooth. Refrigerate. Stir in milk a little at a time if needed when ready to serve in order to thin to desired consistency.

Soak wooden skewers for at least 30 minutes in water before grilling or broiling.
When ready to cook, thread beef, and veggies onto skewers.