>>49300356No, they were added because Sakurai wanted to represent the Kanto starters as a whole with Pokemon Trainer and decided stage 1, stage 2, and stage 3 Pokemon would make a fun theme. Upon deciding Squirtle over Blastoise, and Charizard obviously would be the final form rep, Ivysaur was the choice for mid stage by elimination.
Hundreds of Pokemon have vines and both Bulbasaur and Venusaur are more popular than Ivysaur. Ivysaur was absolutely only picked because of a gimmick/theme