DB and Naruto aren't the only available baseline for defining how a deuteragonist should be, in fact there are plenty of different possibilities and combinations...
Every shonen has its own protagonist-deuteragonist dynamic:
-In HxH Gon and Killua are best friends
-In FMA Ed and Al are brothers and fully support each other
-In One Piece Luffy and Zoro are nakamas and more specifically Zoro is Luffy's partner in crime
So yes, Zoro is not like Vegeta and Sasuke but that doesn't take away nothing from the fact that he's the second main character
Every shonen has its own protagonist-deuteragonist dynamic:
-In HxH Gon and Killua are best friends
-In FMA Ed and Al are brothers and fully support each other
-In One Piece Luffy and Zoro are nakamas and more specifically Zoro is Luffy's partner in crime
So yes, Zoro is not like Vegeta and Sasuke but that doesn't take away nothing from the fact that he's the second main character