Aston Villa He was first scouted by
Aston Villa in November 2016 and then again while representing the United States at the
2017 U17 World Cup. He was finally offered a professional contract in 2018. Vassilev started playing for Aston Villa's U23 team in the
Premier League 2, scoring 4 goals in his first 6 games of the 2019–20 season. He was rewarded with a place in Aston Villa's first team on January 4, 2020, coming off the bench to replace
Marvelous Nakamba in an
FA Cup tie against
Fulham. On January 18, 2020, Vassilev made his
Premier League debut, coming on as a substitute in the 67th minute in a 1–1 draw away to
Brighton & Hove Albion.
Burton Albion On September 17, 2020, Vassilev signed for
Burton Albion of
EFL League One on a season-long loan. He made his Burton debut on September 19, in a 2–1 home victory over
Accrington Stanley.
Cheltenham Town Aston Villa recalled Vassilev from Burton Albion on January 28, 2021. The next day, January 29, he joined
League Two side
Cheltenham Town on loan until the end of the season. He made his Cheltenham debut on January 30, as a late substitute in a 0–0 away draw against
Forest Green. He went on to make 12 appearances for Cheltenham, as they won the League Two title on May 8, 2021 – Vassilev's first trophy.
Inter Miami On July 7, 2021, Vassilev joined
MLS side
Inter Miami on loan for the remainder of their 2021 season. Vassilev made his MLS debut as a substitute in a 5–0 defeat to
New England Revolution on July 21, 2021. On August 8, 2021, Vassilev scored his first goal in the MLS – an injury time winner against
Nashville. He started 7 of 21 games played, scoring 3 goals. Vassilev returned to Aston Villa training in November 2021, after the Inter Miami season had finished. On May 5, 2022, Vassilev returned to Inter Miami for a second season on loan. He started 13 of 24 games played, scoring 2 goals and providing 2 assists. Vassilev was out of contract following Miami's 2022 season.
St. Louis City On November 12, 2022, Vassilev's MLS rights were acquired by
St. Louis City SC as part of the
2022 MLS Expansion Draft. Inter Miami received $50,000 as part of the deal. A permanent transfer for Vassilev to St. Louis City was confirmed by Aston Villa on January 17, 2023, for an undisclosed fee. On February 26, 2023, Vassilev made his debut for St. Louis in their first ever competitive game, a 3–2 MLS victory over
Austin. He started 23 of his 29 games played during the 2023 season, scoring 2 goals and providing 5 assists. In the 2024 season, Vassilev started 25 of 34 games played, scoring 3 goals and providing 4 assists. He rejoined his former
St. Louis City SC manager,
Bradley Carnell, at the Union. == International career ==