Born in Zaragoza, Aragon, Vallejo joined Real Zaragoza's youth setup in 2008, aged 11.[3] On 26 July 2013, after impressing in the Juvenil squad, he signed a new deal with the club.[4]
On 23 August 2014, before even having appeared for the reserves, Vallejo made his professional debut, starting in a 0–0 draw at Recreativo de Huelva in the Segunda División.[5] On 26 December, he further extended his contract until 2019.[6]
Vallejo scored his first professional goal on 5 April 2015, the last in the 1–1 away draw against Tenerife.[7] He was appointed team captain by manager Ranko Popović for that match, and remained in the role afterwards.[8]
Real Madrid
On 31 July 2015, Vallejo signed a six-year contract with Real Madrid for a rumoured €6 million fee, being immediately loaned back to Zaragoza for one year.[9][10] The following summer, he moved abroad after agreeing to a one-year loan deal with German club Eintracht Frankfurt.[11]
Vallejo's maiden appearance in the Bundesliga took place on 27 August 2016, as he came as a late substitute in a 1–0 home win over Schalke 04.[12] He scored his first goal also from the bench, contributing to a 2–2 draw to RB Leipzig also at the Commerzbank-Arena in the last match of the season.[13]
On 7 July 2017, Vallejo was unveiled as a Real Madrid player and member of the first team squad for the upcoming campaign. He was handed the number 3 shirt, previously worn by Pepe.[14] His official debut took place on 26 October, when he started and was sent off in the last minute of a 2–0 away win against Fuenlabrada in the Copa del Rey.[15] His maiden appearance in La Liga occurred ten days later, when he partnered Sergio Ramos in the 3–0 home defeat of Las Palmas.[16]
Profiting from injury to Nacho and suspension to Ramos,[17] Vallejo made his debut in the UEFA Champions League on 11 April 2018, playing the entire 1–3 home loss to Juventus in the second leg of the quarter-finals, which still qualified to the last-four 4–3 on aggregate.[18] It was his only appearance,[19] as Madrid won their third consecutive and 13th overall title in the tournament.[20]
On 27 July 2019, Vallejo moved to English Premier League side Wolverhampton Wanderers on a season-long loan.[21][22] He made his competitive debut on 15 August, in a 4–0 home win against Pyunik in the third qualifying round of the UEFA Europa League.[23] He played his first Premier League match on 14 September, featuring the full 90 minutes in the 5–2 loss to Chelsea also at Molineux.[24] He made one further league appearance, but was not used again except for an EFL Cup tie in a side consisting largely of reserve players.[25]
At the start of the January transfer window, Wolves' head coach Nuno Espírito Santo confirmed that Vallejo was likely to leave and added that the defender "had moments that he played, moments that he performed well, and some moments he didn't perform well ... Clearly it didn't work out".[26] On 24 January 2020, he was loaned to Granada of the Spanish top tier until June;[27][28] on 18 August, the move was extended for another year,[29][30] and he featured 12 times in the Andalusians' quarter-final run in the Europa League[31] for a total of 37 over the campaign.[32]
Due to suspensions and injuries, Vallejo started in the 4–0 home victory over Espanyol on 30 April 2022 as Real Madrid clinched their 35th league title, partnering Casemiro in central defence.[33][34][35] He played only eight competitive matches during the season, however.[36]
On 15 July 2023, Vallejo returned to Granada on a season-long loan.[37][38] He totalled just 106 minutes in his second stint, being relegated as last.[39][40]
^Gaudioso, Sonia (17 June 2015). "Vallejo, con Derecho a soñar" [Vallejo, with the Law/Right (both mean "Derecho" in English) to dream]. Marca (in Spanish). Retrieved 17 June 2015.
^Rodríguez, José María (18 December 2024). "El Madrid corona un año de ensueño" [Madrid crown dream year]. Marca (in Spanish). Retrieved 18 December 2024.