Vallejo started his professional boxing career on June 15, 1990, losing to Pauline Pumarejo by a four-round points decision in Matamoros, Tamaulipas.
Vallejo's first victory as a professional came during his third fight, when he defeated Amado Morado, on September 10, 1991, by a four-round decision at McAllen, Texas, United States.
Vallejo then returned to the United Kingdom, where he faced future world champion Paul Weir on April 27 of 1992 at the Forte Crest Hotel, in Glasgow, Scotland, losing by a second-round knockout.
Next, Vallejo traveled to San Juan, Puerto Rico, where he opposed Josué Camacho for the WBO's vacant world Junior-Flyweight crown, at the Caribe Hilton Hotel.
Among those fights, there was a first-round knockout defeat at the hands of future world champion Alex Sánchez in Camuy, Puerto Rico, and a loss to Paul Oulden at the Eastern Cape, South Africa.