Is it possible to overmarinade a steak? I know it won't spoil; the spoil by date is next week. I'm cooking it in 48 hours (Friday Evening).

It'll rest in the lowest part of my fridge as well.

My marinade consists of vinegar and salt.

Thus the questions: 


Is it possible to over marinade (New York Strip) steaks?  
Is it possible for vinegar to chemically break down the steak too long?
What is the optimal amount of time for vinegar to breakdown the steak?
Yes, the problem will be the vinegar.
Vinegar is acidic and you'll end up with mushy meat. 48 hours is almost certainly too long.  For a vinegar base, I try not to push it over 8 hours and that's only if really necessary. A few hours is typically fine.

Right now, you've got to consider how to save the meat. I'd freeze the meat right now.  Freeze the meat with the marinade (which seems like such a waste for 40'ish hours, but better that than mushy steak). When ready to cook, take it out and let the marinade do its business as it thaws. I'd thaw it in ziplock under running water (quicker than the fridge) and let it marinade a short time and go on the grill. If you let it sit in any unfrozen stage too long, you'll get mushy steak with a vinegar marinade.