Down 1-0 and with 13 minutes of extra-time remaining to find an equaliser, after Fishlock's classy 107th-minute finish, De Vanna took matters into her own hands at Canberra's GIO Stadium.
The Matildas veteran and co-captain snuck up on her former City teammate from behind and rammed an elbow into the back of the unaware Welsh international, who fell to the turf from the impact.
The off-the-ball incident sparked a social media backlash from viewers, who variously described the cheap shot as "bitter", "dirty" and a "soft act".
After referee Rebecca Durcau appeared to miss the elbow, no action will now be taken against De Vanna as the W-League does not have a match review panel.
As is the case in the National Youth League, cameras do not film every one of the women's games, making it impossible to fairly and uniformly apply the system across the board.
It capped a frantic finish to an engaging affair lacking a goal during regular time, despite United's 22 shots to City's 14.
But the reigning champions had the better chances in extra-time and will travel to Perth for Sunday's title decider.