-Don't cook the sauce over high heat; if it gets too hot, it will separate. Better to cook longer over a lower heat and not let it boil.
-I'm not sure if this would work in a slow cooker or not, but you're welcome to try. (Let me know if you do!) Once the sauce gets hot and melts together, stir and leave on the Low or Keep Warm setting until serving so it doesn't get too hot.
-If you don't want to use the frozen veggie blend I mentioned, just steam your desired vegetables until crisp-tender, then drain and proceed. OR cook the sauce and veggies separately, drain the veggies after cooking, then coat with the sauce.