Simon Dunn scored in both halves with George Wells, Lee Togwell and Brad Wadkins also on target as the Rebels booked a meeting with another Hellenic League club in Bracknell Town. Slough will make the short trip to Larges Lane in the New Year.
The opening 20 minutes passed without either side having a clear-cut chance, though Slough looked the more likely with Chris Flood volleying off target and Warren Harris being denied by goalkeeper Curtis Meare.
There was disappointment for the Rebels though, as winger Harrison Gilkes again limped off with a reoccurrence of the hamstring injury that has plagued him since he made his long-awaited injury comeback.
Meare kept out an effort from Flood after being played in by Nathan Webb, and the striker was again inches wide minutes later as Slough piled on the pressure.
The home side took the lead on 28 minutes as a foul on Manny Williams resulted in the award of a spot kick, which Dunn dispatched into the bottom corner.
Two minutes later, Wells added a second, driving the ball past Meare from the edge of the box.
Meare denied Flood again after neat play from Williams, but was powerless to stop Slough making it 3-0 a minute before the interval. Warren Harris broke down the right and pulled the ball across goal, with Togwell on hand to apply the finish at the back post.
Wantage spurned a couple of chances to reduce the arrears at the start of the second half – both efforts going over the bar – but Slough were quickly back into their stride, and threatened another goal on numerous occasions with Togwell, Flood and then substitute Josh Jackman all being denied by a combination of Meare and defensive blocks.
The Wantage goalkeeper pulled off another spectacular save to deny Harris a goal after more trickery from Williams.
Wadkins was given half an hour after replacing Flood in the Slough attack, and he didn’t take long to make his mark, scoring the fourth on 73 minutes from Sean Fraser’s cross.
Dunn added Slough’s fifth five minutes later following a slick team move, with Wadkins dummying for the midfielder to slot home his second of the night as Slough rounded off a comfortable win.
Wantage Town Lineup
Curtis Meare, Dan Comer, Paul Alder, Ryan Markham, Chris Jackson (c), Brad Cox, Jack Powell, Jim Glower, Steve Robertson, Jack Dunmall, Ben Willoughby. Substitutes: Ben Sadler, Jack Heap, Max Dunmall, Mark Flanagan, Josh King.