The 21-year-old defender joined the Lichtenstein club, who play in the Swiss topflight, in January 2014 and has been a star performer when fit.
He had been tipped for a move to a bigger club, with Zurich-based Grasshoppers the frontrunners for his signature, but it has now been confirmed he will remain with Vaduz.
Born in Roodeport, Untersee is a Swiss junior international and a key member of their Under-21 side.
He was identified by Juventus in 2011 and joined their academy that year, but has yet to wear the famous black and white stripes of the Turin club.
He has spent much of the time since playing for Vaduz and will now become the second South African to play in the UEFA Europa League this season after Lebogang Phiri at Danish side Brondby.
Vaduz, who qualified by virtue of winning the Lichtenstein Cup, won their first round, first leg clash against San Marino minnows La Fiorita 5-0.
Coincidentally, Phiri’s club are also against San Marino opposition as they won 9-0 against Juvenes/Dogana.
Untersee has yet to be capped at senior international level by Switzerland, but did play recently for their Under-21 side against Germany and Scotland.