Choose a Gentle Marinade
The use of acidic marinades to tenderize meat is a well-established tradition, but the simple fact is that most marinades do little to tenderize meats. In the case of veal cutlets they're especially problematic because the cutlets are already tender: Not only is their flesh delicate to start with, but pounding the cutlets tenderizes them physically.
Time, Acidity and Texture
Acidity does indeed weaken the bonds between proteins in your meat initially, at least at the surface, but vinegar as a meat tenderizer has limitations. Over time its acidity actually begins to tighten and toughen the proteins, much the same way cooking does. If you've ever made ceviche, which uses an acidic marinade to "cook" the raw fish, you've experienced the same phenomenon. The thinner the piece of meat, the more quickly it begins to toughen, and veal cutlets are usually very thin. Even worse, a strongly acidic ingredient such as white vinegar, with its harsh flavor, overwhelms the delicate flavor of the veal.
Pounding the Cutlet Tenderizes Them
More to the point, any kind of veal steak marinade is really unnecessary. Cutlets are usually taken from a less-tender cut of veal such as the leg, but "less tender" in veal terms is still comparable to a normal beef steak or pork chop. Pounding out the cutlet with a meat mallet or other blunt instrument, to make it thin, is also a superbly effective way to tenderize the meat. The hammer physically tears the bonds between the muscle tissues, permanently tenderizing the delicate veal without damaging its flavor.
Marinating Veal Effectively
That doesn't mean you can't marinade the cutlets, if you wish to. A mildly acidic marinade using milk, buttermilk or plain yogurt does help tenderize thin cuts reliably, and it also provides an opportunity to infuse the veal with appropriate flavors. Thyme, chives, tarragon and many other fresh herbs work well with veal, or you can choose a few flavors from your specific veal recipe and add them to the marinade. They'll leave the veal with just a hint of their distinctive flavors, making it an even better complement for the chosen sauce.