The Finnish starlet was making his first start of the season for the second string and netted after 42 minutes with an excellent finish from a seemingly impossible angle.
The move was instigated by Damien Plessis when his astute pass sent Alex Kacaniklic racing away down the left. His cross found David Amoo whose strike was well saved by Joe Collister, but Dalla Valle was quickest to react and the No.9 scored.
David Amoo and an own goal completed the scoring for John McMahon's team in the second half against a Tranmere side that finished the game with 10 men.
Another great boost for the squad was the return of Spanish starlet Francisco Duran, who made his first appearance since November 2008 when he came on as a 73rd minute replacement for Chris Mavinga.
The former Malaga midfielder has battled back from three cruciate ligament operations - two on his right knee and one on his left.
Also back from injury were Nikola Saric and Gerardo Bruna, who both enjoyed 45 minute run-outs.
The Liverpool Senior Cup holders will now face Everton or Skelmersdale in the final.
After four games postponed, John McMahon's side were playing their first game since February 1, and they were being watched by Rafael Benitez and Sammy Lee before the pair headed to John Lennon airport for their flight to Lille.
Reds heroes Kenny Dalglish and Phil Thompson were also in attendance.
For McMahon it was a match of sentimental value as he spent eight years at Prenton Park having been brought to the Wirral club by Kop hero John Aldridge, and also had a spell as caretaker manager of Tranmere.
Rovers fielded former Liverpool FA Youth Cup winner Charlie Barnett in midfield and had the experienced Chris McCready alongside the returning Ash Taylor in defence.
Playing on a superb surface at the Kirkby Academy, Liverpool quickly got into their stride with skipper Jay Spearing spraying some great passes around the field.
Amoo and Kacaniklic were always dangerous down the wings but the final ball and some good defending from Shaun Garnett's side was proving a tough barrier to break down.
The Reds went close when Spearing's clever header from a corner was hacked away in the six-yard box, but persistence was the key and the breakthrough came on 42 minutes when Dalla Valle pounced.
The second half started in dramatic circumstances when Tranmere were reduced to 10 men three minutes after the break. Aaron Cresswell was shown a straight red card for a late challenge on Steven Irwin.
The Reds then nearly doubled their lead when Collister produced a great save on the stretch to deny Kacaniklic, and Bruna could only put the rebound wide.
Collister was called into the action again on the hour mark when he kept out Spearing's rasping low drive which seemed destined for the net.
However, McMahon's side weren't to be denied and Amoo made it 2-0 with a low finish from the edge of the box.
Dalla Valle could have made it 3-0 but slotted wide after a great move involving Bruna and Kacaniklic. However, a third goal duly arrived courtesy of a Tranmere boot when the Finn combined with Spearing, whose cross was deflected into the net.
Overall, it was an excellent display from McMahon's side and Dalla Valle will take a lot of confidence after netting his first goal for the reserves.
Liverpool team: Peter Gulacsi, Steven Irwin, Chris Mavinga (Francisco Duran 73), Daniel Ayala, Victor Palsson, Jay Spearing (Captain), Nikola Saric (Gerardo Bruna 45), David Amoo (Nikolaj Kohlert 77), Lauri Dalla Valle, Damien Plessis, Alex Kacaniklic. Subs: Nikolaj Kohlert, Martin Hansen, Jordy Brouwer.
Tranmere Rovers: Joe Collister, Tom Baker, Aaron Cresswell, Jay Ashcroft, Chris McCready, Ash Taylor, Ross Rule, Charlie Barnett, Dale Jennings, Conor Roberts, Jay Gibbs.
Liverpoolfc.tv Man of the Match: Jay Spearing. Some fine passing and prompting from the captain.