the lion's digestive and metabolic systems are designed for a high fat/protien diet, and are not designed for a high cellulose diet.
If you can provide the lion with plant based food which was digestable by it's stomach, absorabable by its intestines, and provides the nutrients its bones, muscles, etc required to repair themselves, then yes, a lion could survive physically on plant matter alone.
A mound of grass would not work, because the lion wouldn't be able to digest it. A pile of soy wouldn't work, because it wouldn't provide enough fats, protiens, colesterol, etc.
Could it gather veg. food for itself? No, because most of the food I'm talking about above requires preperation in order to be digestable by the lion. Tofu, wheat glutin, etc, is not available in the wild; it's raw forms, soy beans and weat grain, are not digestable by a carnivorous digestive system.
However, as Dr. Lou suggested, even if the lion is able to physically survive on this veg diet, lions are also designed for the hunt. Even in captivity where all obvious needs are taken care of, the lack of mental reward to tracking and killing prey may result in metal defects in the animal. A short temper, moodiness, general malaise or depression.
And it would have no real understanding of the function of hunting, so if it were ever released into the wild, there is a really good chance that it would starve to death. Sitting around, waiting for it's bowl of miso soup. It wouldn't understand that the elk over there is supposed to be dinner.
This is acommon problem when trying to release captured and injured animals back into the wild. If they become too used to humans and having food provided for them, then they die once released, unable to fend for themselves.