On the heels of his success with Northland, Vainikolo was drafted into the Highlanders squad for the 2008 Super 14 season. While the team endured a difficult campaign, Vainikolo stood out as an offensive weapon, finishing third in the entire competition with six tries, including several of the highlight-reel variety. In the 2009 Super 14 season, Vainikolo struggled to repeat his first-year success, scoring only two tries and coming into increased criticism for his defensive play, splitting starts with the more reliable Kendrick Lynn. In 2010, he started the season sharing playing time with James Paterson, but started the last seven games of the year and finished with five tries to co-lead the club along with Israel Dagg. Concerns about his all-around game and poor form during the 2010 ITM Cup saw Vainikolo left off the Highlanders main squad for the 2011 Super 15 season, although he was included in the team's wider training group.