Ferb and Vanessa's relationship

Ferb has a crush on Dr. Doofenshmirtz’s daughter, Vanessa. He has had a few fantasies about her and has helped her willingly a couple of times. Whether Vanessa has romantic feelings for him or not aren’t clear as she already has a boyfriend. However, the two seem to have a good relationship.

Ferb's Opinion of Vannesa
Ferb first saw Vanessa in “I Scream, You Scream” at Blueprint Heaven when he was picking up a blueprint for an ice cream machine. Their meeting was brief, but at the end of the episode when Ferb is reminded of what occurred, it is revealed that Ferb has a crush on her. Their relationship is briefly seen in “The Chronicles of Meap”, when Ferb is riding in Meap’s new ship, he backs up to Vanessa, who seems impressed by his ride. Ferb still has a crush on her, and is nice to her, helps her, and gives her advice.

While recovering the Pizzazium Infinionite
Their relationship is also later explored in the episode “Vanessassary Roughness”. While at the Superduper Mega Superstore, Vanessa is trying to find pizzazium infinionite to prove to her father that she is responsible enough to own a car. Ferb repeatedly offers his help to Vanessa, even though he is initially rebuffed. When Vanessa was in a life threatening situation with a lawnmower, she called out for Ferb, who immediately saved her. Ferb and Vanessa worked together, and were eventually able to retrieve the pizzazium. Before Vanessa left with her father, she kissed Ferb on the cheek in thanks for helping her, and he went into another fantasy.

While at Paris, France
Later in Season 2, while Phineas and Ferb are traveling the world, Ferb stumbles upon Vanessa in Tokyo, Japan when she falls onto the Sun-Beater 3000 as they few by. They instantly recognized her, and Phineas invites her to go to Paris, France with the group to refuel. While at Paris, they rode together on a motorbike to the Eiffel Tower so Ferb could plot the path to go on their way back to Danvilel. At the top of the Eifel Tower, Ferb gave Vanessa some advice about how to relate to her father. While there, he noticed a flower shop, and went to buy a flower for her. Though when he returned, she was gone. (Phineas and Ferb: Summer Belongs To You)

Vannesa's Opinion for Ferb
When Vanessa and Ferb first met, she just thought of Ferb as another boy on the street. “(I Scream, You Scream)”

While recovering the Pizzazium Infinionite
In "Vanessassary Roughness", Ferb offered her help in her pursuit of the Pizzazium infinionite, she refused at first, but then finally accepted. Ferb saved her life after being nearly being shredded by a lawn mower. She thanked Ferb for helping him, and kissed him on the cheek.

While at Paris, France
In “Phineas and Ferb: Summer Belongs To You!”, Ferb saved Vanessa’s life again on accident. Vanessa went on the plane with Fer, to travel around the world. Once they got to Paris, she let Ferb ride on her motorbike with him. When on the top of the Eiffel Tower, Ferb gave Vanessa some advice on her relationship with her dad. When she saw her dad on Perry’s hovercar, she went with him, thanking and saying goodbye to Ferb as they flew away.

Romantic Moments
In Vannesasary Roughness, Vannesa kissed Ferb, except only on the cheek. In Phineas and Ferb: Summer Belongs to You, they rode a motorbike together, and were together on the top of the Eiffel Tower.