The Tongan-born winger was trapped at the bottom of a ruck midway through the second half and had to be substituted as a precaution.
Having been assessed at Watford General Hospital after the match, Vainikolo underwent follow-up scans on Tuesday morning at The Princess Grace Hospital in London.
Consultant Peter Hamlyn said: "The scans have confirmed there is no damage to the bones of the neck, though Lesley has suffered some bruising to the muscles in that area."
Vainikolo, who joined Gloucester from Bradford Bulls this summer, will be monitored for the next couple of days and will return home on Thursday.
Gloucester's head coach Dean Ryan added: "He has made a fantastic start to his union career and we will obviously miss his contribution on and off the field - but it will allow other members of the squad to push for selection.
"We are naturally extremely relieved that the injury is not more serious.
"We took every precaution necessary at the time of the injury and now we can manage it, it is very good news for everyone."