Vassilev has represented the United States internationally at a number of youth levels and made his senior debut in January 2025.
His coach at the IMG Academy, Kevin Hartman, described him as having great leadership qualities and all the attributes he thought were required for him to play at the top level.
[4] 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.
[5] 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.
[21] Due to his parents' nationality, Vassilev was eligible to represent both the United States and Bulgaria at international level.
[24] On October 8, 2023, Vassilev was called up to the United States under-23 national team ahead of friendlies against Mexico and Japan.