Highlanders coach Glenn Moore said Vainikolo would have to be watched over the next couple of weeks and would be on restricted duties as he tried to repair his damaged hamstring.
The injury occurred over the Christmas break when players had to do their own training, and Vainikolo could manage only a light jog at training when the players regathered on Monday.
The 23-year-old Tongan flier was a stand-out for the Highlanders last season, his first year in the Super 14, and will be hoping for another top season this year.
Another outside back in the squad, Ben Smith, was making a good recovery from surgery on a nasty foot injury at the end of the season last year.
In something of a curse of the outside backs in the return to training, centre Jason Shoemark and winger Lucky Mulipola were unable to train yesterday because of illness, but were expected to be back on deck today.
Smith had his first full run and change of direction on his foot this week and Moore said it was looking promising to have him available for the final pre-season game against the Blues at North Harbour Stadium on February 5.
Moore said he was very pleased with the work the players had put in over the past couple of weeks, and the side was well ahead of where it had been at the same time last year.
"You always think you would have liked to have done more but we are well ahead of where we were at at this time last year," he said.
"We have done a lot more skill work and game-related stuff."
Moore said there was a good resolve within the group, and a good spirit developing.
The side would train on Carisbrook on Friday, with Moore saying he wanted to introduce players to the ground.
They would train occasionally at the ground during the season build-up.
He also hoped to carry out some trainings at schools around Dunedin to help build support for the team and get the players to connect with supporters.
The side's first pre-season game is against the Chiefs at Rotorua on January 23.
The other pre-season match is against the Crusaders in Timaru on January 30.
