He made his Super League debut for Olympiacos on 16 April 2018 in a home game against Kerkyra. Vrousai entered the court of Karaiskakis Stadium in the 78th minute of the game, as the third substitute of Olympiacos' interim coach Christos Kontis, taking the place of Belgian international Kevin Mirallas. Nine minutes later, he scored the first goal of his professional career, after receiving an assist by team's striker Karim Ansarifard. Vrousai made three more Super League caps until the end of the season and was considered as one of the most talented players from the youth academy of the Greek club. On 9 August 2018, he made his debut in the European competitions during the UEFA Europa League third qualifying round against FC Luzern. Olympiacos got a 4–0 victory versus the Swiss team at Karaiskakis Stadium for the first leg of the round and Vrousai played as a late substitute for the last 12 minutes of the match. Portuguese manager Pedro Martins replaced Lazaros Christodoulopoulos with the 20-year-old Vrousai and gave him the chance for his first ever European appearance. On 19 December 2019, he scored an amazing long shot goal, the first of Olympiacos in the 1–2 road victory versus Aris Avato at Xanthi Ground for the last matchday of the Group Stage of the 2018–19 Greek Football Cup.
Loan to Willem II
On 16 January 2019, Vrousai hasn't been counted on by Pedro Martins, and he has been allowed to join Dutch club Willem II on loan, for the remainder of the season. In accordance with the agreement between Olympiacos and Willem II, Vrousai is set to stay on loan with the Dutch club until the completion of the 2019–20 season. If Willem II wish to sign Vrousai on a permanent basis, the release clause amounts to €1 million, while in the event Olympiacos wanted to end his loan prematurely this summer, they would have to pay the Eredivisie team a fee. A lot also depends on the player's personal wishes. Olympiacos would be able to re-sign Vrousai for €2 million in the future if Willem II activated the release clause of one million. On 20 January 2019, Marios Vrousai scored in his debut with Villem's jersey, sealing a 2–0 home win game against Breda after Vangelis Pavlidis assist. On 2 February 2019, he scored after an assist from Daniel Crowley in a frustrating 1–2 home loss against FC Groningen. On 24 February 2019, he opened the score with a header in a 2–1 home win game against AZ Alkmaar. On 18 August 2019, in the third game of the 2019–20 Eredivisie Vrousai scored with a good header, in a 2–3 away win against Fortuna Sittard. It was his first goal for the season. On 19 October 2019, he successfully connected with a floating cross from the right wing, planting a firm header beyond Twente goalkeeper Joel Drommel, after an assist from Freek Heerkens, sealing a vital 1–0 away win against FC Twente. On 27 October 2019, Vrousai will be out of action for 4–6 weeks after being hit in the ankle during Willem's training. On 23 November 2019, he returned to the squad, playing as a substitute in a vital away 3–3 draw against ADO Den Haag.
