Why meat gets tough after soaking it in vinegar solution for more than 24 hours?

Vinegar is acidic thanks to its concentration of acetic acid. It thus has a low pH, somewhere between 2.5 and 5 depending on the type of vinegar. Acidic substances can affect the delicate amino acids which form the proteins that make up the majority of your meat (along with other fats and connective tissues).

The end result is not entirely unlike what happens when you apply heat. The proteins in the food change shape and coagulate, whether you're applying heat or acid, and that results in "cooked" meat. It's the principle behind many cures or preparations like ceviche that use chemical action to alter a food from its raw state. The end result often has less moisture; thus it appears firmer and can be perceived as tougher.

The other thing happening is that a strong marinade is much more concentrated than the water inside the cells of the food being marinated, and thus will tend to extract some water through osmosis. Again, you wind up with drier, tougher meat. Most marinades compensate for this to some degree because they contain salt, but 24 hours is a pretty long time for a strong marinade containing vinegar, so that may just be overdoing it.