[1] He made his senior debut with the reserves on 25 January 2015, coming on as a second-half substitute in a 2–1 Tercera División away loss against Atlético Sanluqueño CF.

Vallejo made his first team debut on 13 January 2016, playing the last 30 minutes in a 2–0 away loss against Celta de Vigo, for the season's Copa del Rey.

[6] He scored his first professional goal on 12 September 2018, netting the winner in a 2–1 away defeat of CD Tenerife, for the season's Copa del Rey.

[8] On 7 February 2019, La Liga side Valencia CF reached an agreement with Cádiz to sign Vallejo; the player remained on loan at the Amarillos until June.

[11] Vallejo scored his first goal in the main category of Spanish football on 21 December 2019, netting a last-minute equalizer in 1–1 away draw against Real Valladolid.