DB Heroes doesn't follow the manga exactly. It takes inspiration and builds on it, making changes to suit the specific requirements of the series. However, the core elements are still recognizable.
Vegeta is also very likely to be a hero in the new story. He has grown a lot throughout the Dragon Ball series and in DB Legends, he might play a crucial role in fighting new threats. His Saiyan pride and his determination to surpass Goku could drive the story forward in interesting ways.
For Yamcha, you can focus on his growth from being a bit of a comical character to a more serious and respected fighter in the context of his relationship with Frieza. Maybe he learns from Frieza's cold and calculating nature how to better strategize in battles. For Frieza, his development could be about him learning to care for someone other than himself. He could start to see Yamcha as an equal, which is a huge shift from his usual superiority complex.
You can also make him have a unique role in the story. For instance, he could be the one to discover a new threat to the Earth before anyone else. He then has to convince the other Z - Fighters to take this threat seriously. This would put him in a leadership position and make him stand out more.
The length of DB mangas differs. It depends on factors like the story's complexity and the author's vision. Some might run for just a few volumes, while others could have dozens.