Ex-world champion Kimi Raikkonen and even Vettel's long-time rival Fernando Alonso had also been linked to the Red Bull vacancy, with Webber retiring from Formula One.
"We won't say anything before Spa, that's all I can say," Red Bull advisor Helmut Marko told Sport Bild.
Ricciardo has been sponsored by Red Bull since 2008 and he started driving for Toro Rosso last year, claiming 21 Formula One points from 41 Grand Prix races.
If the deal goes ahead, the Australian will follow the path Vettel took in 2009 when he switched from Italy's Toro Rosso to her British-based sister team Red Bull Racing.
With ten races gone in the 2013 season, Vettel leads the world drivers' championship table with 172 points head of second-placed Raikkonen on 134.