The vice presidency has been a largely symbolic role in previous decades, but it is expected to be adapted to allow Wang to become involved in issues chosen by Xi, even though he is not a senior member of the Communist Party.
After the congress's repeal last Sunday of the constitution's limits of two five-year terms for the presidency and vice presidency, Mr. Wang could stay in office indefinitely, as could Mr. Xi.
Wang's appointment shows that "he's a really important political advisor", said Kerry Brown, director of the Lau China Institute at King's College London.
This comes after last week's decision to lift term limits on the offices of president and vice president, meaning that this will probably not be the last time Xi lifts his fist to take the oath.
"Our Chairman Xi is too great, truly he is too great", said Du Meishuang, a Chinese opera singer and delegate from Chairman Mao's home province of Hunan.
"The "fear factor" will ensure that Xi will have his way and Wang will be grudgingly accepted for whatever role or roles Xi may assign to him, he said".
The congress reelected Xi as president with a vote of 2,970 for to zero against. He also has an extensive network of influential global contacts and forged enduring ties with major Wall Street figures such as Hank Paulson.
Wang was at the frontline of Xi's anti-corruption crusade, heading the party's Central Commission for Discipline Inspection, which has punished 1.5 million officials in the past five years, from low-level cadres to regional leaders and generals.
Mr. Wang, the only candidate on the ballot for vice president, received all but one of the votes from the 2,970 delegates to the National People's Congress inside Beijing's Great Hall of the People.
Wang retired from the Politburo of the Communist Party a year ago at the party's customary retirement age of 68.
Wang, once the party's formidable anti-graft chief who has mostly worked behind the scenes, read out his oath in a gravelly voice, slowly but confidently. A new supervision law, set to be approved Tuesday, will broaden his campaign against corruption beyond party members to include all public-sector workers. Among Mr. Wang's priorities will be helping Mr. Xi manage the critical but increasingly fraught relationship with the US, people familiar with the matter have said.