Might as well learn to make a pincage, eh?2 parts beef stock to 1 part chicken stock = Fast and dirty approximation of veal stock, no. 1.
1 part beef stock to 1 part roasted chicken stock = Fast and dirty approximation of veal stock, no. 2.
1 part beef stock to 1 part chicken stock = Fast and dirty approximation of pork stock.
That having been said, I'm with whoever said they've never seen a chicken marasala recipe calling for veal stock. I think it would completely overwhelm the chicken -- especially breast.
If you find chicken stock leaves too light a color (it does), you can get a little color and structure by using what's called a pincage. If you've never done this, you should. It's an important technique.
If you make marsala like most people, you'll brown your chicken first, nearly cooking it through, then remove and reserve it. Add your shallots (if you use them) and mushrooms to the oil and fond, and put a little color on them. That accomplished, add a little bit of tomato paste to the hot pan -- no more than a tsp. Let the paste form a light fond of its own on the pan, a matter of a couple of minutes at most. Then start moving it around until the paste has darkened, nearly turning brown -- two or three more minutes. That's the pincage.
To continue, sprinkle a bit of flour, say a tsp, on to the pincage and keep moving the contents of the pan around until the flour no longer smells raw. Then you can add your marsala, stock and herbs. When the sauce comes together, return the chicken to the pan and let it reheat.
TIP: A better alternative to adding flour to the pincage is dusting the chicken with flour before browning. That will leave enough flour in the fond to structure the sauce, along with the tomato paste. FWIW, that's my preferred method.
One other thing -- if you're making this for yourself, and not for money -- try using boned out thighs rather than breasts. More flavor, and they hold up better without drying out as quickly. In fact, as a general matter, thighs are better in veal presentatoins than breasts.