The 23-year-old was strongly linked with a move to the Premier League in August, but a reported move broke down at the final moment.
"There are no agreements or promises made. I can imagine that they will continue to monitor Vadis, but there are no guarantees. If Everton want Vadis in January, the negotiations will start from scratch," Mannaert told Sky Sports News.
Everton sit sixth in the Premier League table.