Sanji loyal to his crew and his captain(he would not follow luffys orders IF A CREWMEMBER IS ON DANGER)
Zolo only loyal to his captain
Now who can be the vice captain? Someone who do his best for all of the crew or someone who follow blind the orders from his captain.
What if luffy is not clear by his mind and orders zolo to attack nami, cause someone hypnotositic luffy? Sanji would not attack someone from his crew and zolo would.