The former Bradford Bulls rugby league star reappeared in Gloucester colours last month following the death of his brother.
But Vainikolo last night flew back to deal with what Gloucester head coach Dean Ryan has termed "a serious family problem".
Vainikolo, the Premiership's leading try-scorer this term, and fellow wing James Simpson-Daniel will both miss tonight's top-of-the-table Guinness Premiership clash against west country rivals Bath at the Recreation Ground.
England international Simpson-Daniel has flu, meaning call-ups to the starting XV for James Bailey and Leon Lloyd.
Ryan said: "James reported this morning that he was really not very well at all, and last night we were informed of a serious family problem in New Zealand that Lesley needed to attend urgently.
"Naturally, our thoughts are with him. We will give him as much time as he needs and support him on his return."
Bailey has started three Premiership matches this season - against Leicester, Sale and London Irish - while former Leicester back Lloyd has featured eight times in all competitions.
Leaders Gloucester go into the game with a five-point advantage over Bath, having won eight of their nine Premiership games this season.