The 18-year-old had previously agreed to join the Seagulls in January from Romanian outfit SCC Farul last summer, and continued to play in his homeland on loan for the first half of the campaign.
Mazilu has now made his transfer to Brighton official, but he will immediately make the temporary switch to Eredivisie side Vitesse where he will hope to gain valuable first-team experience.
Speaking to Brighton's official website, technical director David Weir said: "We have been monitoring Adrian and he has done well in Romania. He now has a new challenge with Vitesse in a new league and environment, and we will continue to track his progress closely."
"This is an incredibly great opportunity for me. Vitesse is a wonderful club," Mazilu told Vitesse's official website.
"Of course I hope to be important for the team in the coming months. In any case, I'm looking forward to it and I'm going for it with full energy."
Vitesse technical director Benjamin Schmedes added: "The wing position was an absolute point of attention. Especially with Miliano Jonathans' long-term injury in mind.
"Adrian is a talent and has already gained the necessary experience despite his young age. We are very happy that he will be joining Vitesse for the second half of the season and we wish him a warm welcome."
Mazilu made his senior debut for boyhood club Farul under the tutelage of Romanian icon Gheorghe Hagi in November 2022, and he enjoyed a breakthrough year with the club during the second half of last season.
The Romania Under-21 international departs Farul having made 46 senior appearances across all competitions, scoring nine goals and registering four assists.
Mazilu will hope to make an impact in the first team at Vitesse, who currently sit rock bottom of the Eredivisie table and are seven points adrift of safety.
Brighton, meanwhile, sit eighth in the Premier League table and Roberto De Zerbi's side could climb above Manchester United into seventh place if they beat Wolverhampton Wanderers on home soil on Monday night.