The 24-year-old winger always had it in his mind to return to the 15-man game which he played as a youngster in Fiji and decided the time was right when the Queensland Reds came knocking with a substantial offer.
Vunivalu reportedly signed a deal in late November worth $1.8 million with the Reds and Rugby Australia.
While he didn't comment on the size of the deal, he said hamstring issues, which ruled him out of the Storm's painful preliminary final loss to Canberra last year as well as the World Cup Nines, helped him make the decision.
"It was a hard decision as I've been here six years and wanted to continue but I've got to do what's best for me and my family," Vunivalu said.
"I wasn't going back this early but I thought that most rugby players don't play for that long and injury-wise I've had a couple of hamstrings so I thought it was the best offer for me and my family."
As well as Vunivalu, fellow flyer Josh Addo-Carr will also depart at season's end but the pair hadn't spoken about it being their final season.
"We haven't concentrated on leaving, we've got our final year to make the most of it," Vunivalu said.
"Our focus will just be on the Storm and nothing more than that."
Vunivalu will have his first hit out of the year in Melbourne's final NRL pre- season trial against North Queensland in Cranbourne on Saturday.