As we know, how the food source was raised, and the quality of the fertilizer (for plants), and feed (for animals) is important for the final quality of the food produced. You may be right that Soylent green would be healthier than soy, but not by much, given the quality of "feed" for the animals used. Especially down the road, when the "feed" to produce soylent green is, in fact, soylent green.
Because we may know that soylent green "is people", but we also know, not entirely; high tech is in there too. Certainly not just sustainably-raised-free-range-species-specific-properly-fed people.

ETA: Sorry, just took in the title of this thread. Soylent green is obviously NOT vegan.