Why Brook versus Vasco? And why would Sabo fight alongside the SHs in their crew vs crew battle?
Vasco shot was introduced as a kinda tertiary BB pirate along with Sanjuan Wolf and Catarina, who make sense as Franky and Robin fights respectively.
As such, Brook vs Vasco works character wise as the adult trio are important but never as important as the other straw hats. Not to mention, Vasco appears to be a jester with his design. Honestly Brook is the only straw hat who lines up at all with Vasco.
As for Sabo vs Burgess, I think the fight set up has been done.
1. Sabo and Burgess competed over the flame flame fruit in which Sabo was able to win
2. Burgess then snuck aboard a Rev ship and lead the BB pirates to destroying Sabo’s Home
3. Sabo is currently separated from the revolutionary army for an unknown reason
4. Sabo has Luffys vivre card and is very likely to play an Oden type character on Luffys ship (Luffys brother, wants to write a book of his adventures, is the number two of a separate group, royalty who refused his position and hung out with punks in the lower town instead)