i should add that, as long as the hips/knees are good or excellent, I don't mind my seniors carrying a few extra pounds. years ago we had a lab who was dysplastic and I was very careful she NEVER carried extra weight. Jet's ideal weight is about 26 and he maybe a few pounds overweight. It's a little more difficult for the seniors to bounce back from any sort of illness and Jet's hips were OFA'd Good, so I don't worry about a little extra.
The veggies will add fiber to the dogs' meals and help fill them up. I would not use canned as it tends to be high in sodium. If they don't like the veggies, you may want to try to cook them in a little chicken broth to help the flavor. or you can decide if they aren't hungry enough to eat the veggies, they aren't really hungry.
My house is colder in the winter, so I tend to feed the b's 1/3-1/2 a cup 2x a day. And I tend to train more during the nicer days, so Z gets very little at meal time as I have to watch that. (I use lots of treats during training.)
But really, I would not switch to a different food just to reduce the weight. For Z I also usually use a puzzle toy for feeding her meals so it lasts longer.
For comparison, Zest!'s official measurements (for AKC agility) were 15.88 and 16 inches and I think she looks best at 20 pounds.