A few thoughts:
a) Yes, vegetarianism was THE diet of our forefathers. At that point, death had not entered the world, so animals were not part of the pre-fall nutrition system.
b) Why vegetarianism? Because it was the first creation, pre-death and pre-fall. All things were given to Adam and Eve for their enjoyment and consumption, and animals were not necessary to provide the satisfaction Adam needed. It is true animals were under the dominion of man, but in a different capacity. Therefore, Adam named them accordingly.
c) After the fall, animals–rather than partners in the garden–became fully subject to man’s dominion. Before the fall, there was no need for food regulations. After the fall, food regulations, cleansing laws, ceremonial laws, all became necessary to distinguish God’s people from those outside the covenant.
d) Was it God’s original design for men to be vegetarian? My answer is No. But I am assuming God knew the Fall would happen. His intention was always to provide meat for his children. And through Jesus He cleanses all foods (Mark 7:19).
e) Final analogy: When our first parents were created, they were babies, even with the appearance of age. When Jesus came He came to usher a humanity of grow-ups, mature people of God. When He came, he cleansed all foods, and gave grown-ups the privilege of choosing one food over another, or both. With Christ, humanity increases in responsibilities. In the Garden, Adam and Eve had simple tasks and their diets were simple, but Christ rescued humanity from the fall, and gave them greater responsibilities in this new garden He is cultivating called the world.