In the past,
Yangus himself promised to obtain the Venus' Tear in hopes of impressing his fellow thief and friend,
Red. Though he originally feared the labyrinth and was unsuccessful in obtaining the gem, he found himself following through on his promise as part of
the Hero's party when their friend
Medea (trapped in the form of a horse) was obtained by Red through a series of events in
Pickham.
The party succeeded in retrieving the gem, and presented it to Red to rescue Medea.
If you get the gem before visiting Red, Yangus and the party will not present her with it until you leave Red's House and return to it. Storywise, Yangus explains to the party that in order for them to get Medea back, Red must believe that they obtained the gem for her benefit rather than lucking upon it.
Red holding the Venus' Tear