Within the original Final Fantasy VII, Yuffie Kisaragi and Vincent Valentine were optional party members, meaning you could potentially complete the game without recruiting them. While Yuffie took the starring role in VII Remake Intergrade's DLC, that approach will likely change in the upcoming sequel.
The DLC's left some fans wondering about Vincent's status, and during an interview with TheGamer, Final Fantasy VII Remake's co-director Motomu Toriyama weighed in. Giving us a fairly straightforward answer, he confirmed Vincent's "canonically sleeping".
Final Fantasy VII Remake's DLC Chose Yuffie Over Vincent Because He's Currently Sleeping
Detailing this further, Toriyama-san specifically confirmed:
Yuffie, the heroine of Episode Intermission was not guaranteed to join your party in the original game, but that actually made her a character that we had more freedom to expand on the backstory for. Compared to Vincent, who was canonically sleeping in a coffin under the Shinra Mansion and so could not be moved around, Yuffie was journeying around the world as a materia hunter.
By inserting the events in Midgar from FF7R Episode Intermission into that journey, we can show the feelings that she had towards that incident that are usually hidden behind her cheery personality, and that will tie into her story further down the line.
Though VII Remake's shown Square Enix aren't afraid to change elements of the original continuity, it's a reasonable explanation for Vincent's current absence. It'd be easy to assume that VII Remake plans to make Vincent and Yuffie mandatory characters, though for now, that remains speculative. As always, we'll keep you informed when we learn more.