vice-president of a political party; deputy party leader
Refers to the second-in-command of a political party. The exact title and role may vary by party and country.
He was elected vice-president of the party.
The deputy party leader explained the party's policy at the press conference.
副総裁 is a more general term for vice-president of an organization, not limited to political parties.
Compound of 副 (vice-, deputy) and 党首 (party leader).