Sometimes you have to set limits with your Vizlsa. Mine are not allowed in the kitchen,or dining room, during dinner, or meal preparations. No matter how well trained, sometimes the temptation is to great. We also never feed treats from the table or from the counter.
Female dogs of all breeds can be more intensive with regards to resource guarding, than males, so the end resultant behavior you observed doesn't surprise me. The level of aggression does though, especially for a dog that has been with you daily for 2-1/2 years. That is surprising.
Through the years I have always tried to condition my dogs to not be food aggressive. It's a time consuming process, and not a 100% guarantee, but it can reveal certain behaviors.
One of my Vizslas was not food aggressive at all. I could put my hands in his bowl, and regardless of what was in the bowl he wouldn't bite at my hands. I'd take food out of bowl while he was eating, and then put it back. with never a problem. Steak, chicken, ham, bacon, it didn't matter. One night though a prime rib bone from a restaurant set him off. I would have never expected it. He was dead serious that he wasn't giving up that bone. We went a few rounds n the kitchen with give and give back drills with that bone, but after that we just put him inhis kennel if he got a bone with meat on it, and closed the door.
Make sure your poochie is okay, and doesn't have Lyme, then start working with her on taking food and giving it back.
Limit her from the kitchen during dinner and try, to the best of your ability, to not have her get food from the kids. ( Better the trials of Hercules than getting dogs and kids to stop sharing food.
) In the end though, she may just need to be put away for dinner time.
Good luck. If you want to know how to condition the dog against food aggression ,post back and I'll try to outline the method I use.