Zoro would need named moves to damage the Raid Suit, so Sanji would be able to hold clashes with his blades in normal exchanges.
He would need to dodge Zoro's powerful moves, as most non swordsman would with lethal attacks, or divert them.
He has superior maneuverability, speed and CoO, he'd be able to land some of his own attacks better than Zoro.
He can disengage better than Zoro at will, and he can use the RS boosters to fly around without getting tired at all while Zoro consumes stamina to throw slashes at him.
He can abuse invisibility which was shown to be effective on the likes of King and Queen, who only noticed Sanjis presence when the target they were protecting started flying around.
Obviously Zoro has his own advantages in CoA, lethality, destructive power, can be very fast in short bursts and has good reaction time. Also Oda would showcase his endurance and will power in such a fight, so even if Sanji damages him he would certainly persevere and pull out the win.
Like i said, if Oda writes Zoro vs Sanji, Zoro wins in the end after a hard fought battle, where both combatants acknowledge the strength of their opponent, unlike their biased fans who like to shit on one or the other