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.
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.