A late Ihaia West try ended Racing 92's stiff resistance as La Rochelle booked their place in a second successive Champions Cup final.
Ronan O'Gara's side, who were beaten by Toulouse in their debut Champions Cup final at Twickenham last season, will face Leinster in Marseille on 28 May.
Hosts Racing had to relocate the game to Lens as their La Defense Arena in Paris was being used for a music concert.
Le Garrec nudged three-time runners up Racing in front, before France centre Vakatawa showed his international experience to step back against the traffic and ground the ball for the opening try.
West missed what seemed a simple penalty and the visitors were slow out of the blocks but eventually France number eight Alldritt drove over Le Garrec in a physical mismatch to reduce the deficit to two points.
Another missed penalty for West failed to reward La Rochelle's recovery after the break, as Le Garrec successfully kicked Racing further in front.
But the pressure on the Parisians began to tell as Chat was sent to the bin for being off his feet at the breakdown.
A minute later, Racing were down to 13 players as Gomes Sa joined Chat on the sidelines for side entry at the maul, in a move which resulted in a penalty try to hand the visitors the lead for the first time.
As the hosts tried to respond, they surged deep into La Rochelle territory but Teddy Thomas opted to take contact rather than release the pass which would have surely meant a run-in for Juan Imhoff.
With the clock into the red, La Rochelle showed the clinical edge Thomas was missing as the ball was moved wide to the unopposed West to shrug off his disappointment from the tee.
| Racing 92 (10)13 |
| Try: Vakatawa Con: Le Garrec Pen: Le Garrec 2 |
| La Rochelle (8) 20 |
| Try: Alldritt, penalty, West Pen: West |
