The incident, in a Dunedin bar early last Saturday, is being investigated by the New Zealand Rugby Union.
Police confirmed Vainikolo (24) has not been arrested. He declined to comment when contacted last night.
A member of the public had made a complaint to the Highlanders about Vainikolo after an incident, believed to have occurred about 4am.
Reid declined to give details but said he was "bloody disappointed".
"If you're out that late, then you put yourself in a position when things can happen. It doesn't matter where you are or who you are, if you are out late, things have a habit of going pear-shaped," Reid said.
The Highlanders have been plagued with controversy off the field in the past year.
Halfback Jimmy Cowan was involved in three incidents last year, being convicted of two disorderly behaviour charges, while a third charge was dropped.
Wing Lucky Mulipola was convicted of an assault in Dunedin last April and convicted on two driving charges.
Both Mulipola and Cowan were placed on alcohol bans.
Loose forward Adam Thomson is facing an assault charge after an incident in Havelock North in December.
The charge will be the subject of a pre-depositions hearing on February 26.
Thomson has not entered a plea.
Reid said there was not a drinking culture in the team.
"There are team protocols in place but we don't have bodyguards going round keeping control of them. They should be responsible and look after themselves."
He was not planning on introducing curfews for the side, or banning drinking, and said players had been made aware of the expectations of coaches and management.
"Most guys are very responsible at looking after themselves. That is why it is disappointing these things happen."
New Zealand Rugby Union general manager professional rugby Neil Sorensen said the union was taking the matter seriously and had commenced a misconduct process, together with Highlanders team management, which would ascertain the facts and determine any penalty deemed appropriate.
A misconduct hearing is scheduled this week.
Vainikolo would continue training with the team, for its game against the Hurricanes in Wellington on Friday night.
