The 27-year-old lock, formerly with Harlequins for the two seasons before his arrival at Parc y Scarlets, will join an Argentinean development side competing in the Vodacom Cup with immediate effect.
Vallejos joins Pampas XV to compete in the annual Vodacom Cup in South Africa - a competition that has been running since 1998 - recognised as an important developmental competition with 14 South African teams and selected invitational teams.
Vallejos, who has appeared eight times for the Scarlets has agreed a release from his Scarlets contract early to allow him to get full game time in the competition which runs for several weeks in March and April.
Born in Rosario, Vallejos has represented Argentina in the U17, U18, U20 and U23 age groups as well as achieving his first cap for the Pumas after featuring for his country in the 2011 Rugby World Cup.
Scarlets Head Coach Simon Easterby said: "We have supported Tomás's wish to play with Argentinean side Pampas to allow him to get a substantial period of game time which he has unfortunately not had with us.
"Tomás joined us later in our season after his commitments with the Championship and had to play catch up to an extent within the squad dynamic."
Easterby went on to say: "Building up depth and creating strong competition for places is healthy for our region, however it has meant that Tomás has not had a great deal of game time with us.
"We appreciate his wish and respect his decision to move back with an Argentinean side and thank him for the spirit and enthusiasm he has shown for Scarlets rugby during his time with us. We wish him all the best in the Vodacom Cup and his future aspirations in rugby."
Vallejos said he was grateful to the Scarlets for allowing him to take up the opportunity with the Pampas XV so promptly and acting decisively to release him from his contract.
Vallejos said: "It is important to me to be able to take up this opportunity as the Vodacom Cup will ensure that I get some more rugby under my belt which is what I need right now. I've not made any firm decisions about what I intend to do after the Cup competition in South Africa and there are a number of options available for the future."