Wales will be without key tighthead Adam Jones for the Cardiff encounter as the Ospreys prop has a knee injury.
The loss of the 2009 British and Irish Lion is a major blow to the Welsh scrum, which Jones has anchored for much of the last decade.
Wales interim head coach Rob Howley is faced with the dilemma of whether to hand Jones' Ospreys deputy Aaron Jarvis a first cap, or switching loosehead Paul James, who has impressed for Bath so far this season, to the number three shirt.
The decision is particularly vital given the Pumas renowned scrummaging prowess, and they are high on confidence after pushing Australia and South Africa close during their maiden Rugby Championship campaign.
Vallejos, 27, who recently moved to Wales with the Scarlets, says the visitors' approach to the scrum has not been changed by Jones' absence, but they will be looking to exploit any signs of a creaking Welsh forward effort.
"We have a massive belief in our scrum, it is one of the foundations of the team," said the former Harlequins forward, called into the squad as a replacement for the injured Patricio Albacete.
"If Jones plays or not it will not change our approach. We will still believe in our scrum and it will play a massive part in the game for us and we know Jones has been a massive player at the top level."
During his brief stint at the Scarlets, having only joined up with the region three weeks ago after being involved in the Rugby Championship, Vallejos admits he has been impressed by the abilities of some of his team-mates - none more so than George North.
He said: "George is a big player, the thing that surprised me is he is a massive guy and he is physical, but he also has skills and it makes him a very special player.
"I will be telling some of our guys in the backline we need to keep a close eye on him."
But Vallejos hopes he will be able to return to the Scarlets enjoying the bragging rights after this weekend's contest.
"It would be fantastic to win the game especially with me playing here, it would be funny for me to be able to do some jokes in the changing room and I am really looking forward to it."
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