Vassell made significant strides in his game between his freshman and sophomore seasons at Florida State, propelling him to the first round of the 2020 NBA Draft. As a freshman, the 6-6 wing was a role player, averaging 4.5 points in 10.7 minutes across 33 contests. Everything ramped up in 2019-20, however, as Vassell became a full-time starter and increased his playing time to 28.8 mpg. He used that opportunity to produce 12.7 points, 5.1 rebounds, 1.6 assists and 1.4 steals. Vassell's best showing of the year came in a Jan. 18 contest against the Hurricants, where he put up 23 points, 11 rebounds and five assists. Three games later against Virginia Tech, he put up a collegiate career high with 27 points on 8-of-10 shooting. While the stats weren't necessarily eye-popping, Vassell's stock continued to rise due to his efficiency. He finished sixth in the ACC in PER, and was first in the conference in turnover percentage. The Seminoles finished that season with a 26-5 record and were in line for a No. 2 seed in the NCAAA tournament, though it was ultimately cancelled due to the COVID-19 pandemic.