Vaz, 19, joins on a season-long loan with a non-obligatory option to buy, six months after Roma paid Marseille around €25million for the France youth international in January.
The forward managed just 14 appearances and one goal for the Serie A club, scoring the winner against Lecce and providing two assists in a Europa League last-16 tie against Bologna, but never established himself as a regular under head coach Gian Piero Gasperini.
AS Roma will receive an initial €2million for the loan, with Hull City holding the option to make the deal permanent for a further €30million at the end of the season should the teenager's early promise in the Premier League translate into consistent form.
Vaz began his career at Sochaux and Marseille's academy sides before breaking into Marseille's first team, where he scored four goals in 22 appearances and helped the club finish second in Ligue 1 last season.
Hull also confirmed the permanent signing of England Under-21 international Brooke Norton-Cuffy from Genoa for a fee in the region of €14million, with the 22-year-old signing a contract until 2031.
Norton-Cuffy made 46 appearances across two seasons for the Serie A club after joining from Arsenal in 2024, scoring twice, and returns to English football with Premier League experience gained during loan spells at Rotherham United, Coventry City and Millwall earlier in his career.
The two deals were struck independently but form part of a broader swap arrangement between the clubs, with Genoa bringing in right-back Cody Drameh from Hull for around €4.5million as a direct replacement for Norton-Cuffy on the same day. For newly promoted Hull, adding proven Serie A experience at both ends of the pitch reflects an ambitious approach to squad-building as the club looks to establish itself in the Premier League after promotion.
Vaz's departure brings a swift end to what had been billed as a significant investment by Roma when he arrived in January, with Gasperini having always been cautious about the pace of the teenager's development.
Norton-Cuffy's move, by contrast, represents a more straightforward homecoming for a player who has spent his career largely in England's lower divisions and Serie A, and Hull will hope his athleticism and attacking instinct from full-back translate quickly to the physical demands of the Premier League.