In New Threat Vincent is no longer an optional character. I did the completionist run of the Shinra Manor, so I do not quite remember how the mod gets him into the party, but he joins up after Sephiroth yeets the materia at Cloud and flies off.
One New Threat feature I forgot to mention is that each character has an Innate Ability in combat. Taken from the text documents in he mod, here's the innates of all the party members so far:
List of Innates
Cloud: Set up by using the Defend command, then continue fighting as normal. The next time Cloud is hit by physical damage he will counter it with either a single physical attack, a Deathblow, 2x-Cut, or 4x-Cut. This works in addition to any other counter materia.
Barret: When hit by a physical attack, if in the front row he gains Strength (up to +40). If in the back row, he regains 1/16 of his HP.
Tifa: Once per battle, she will survive a KO with 1 HP (or ignore a insta-KO attack) and retains any queued actions, buffs, Limit Gauge, etc. However she also retains negative statuses so Poison or Dual-Drain will undermine this innate.
Aeris: Activate by Defending, Aeris will absorb most elemental damage and HP while she defends.
Red XIII: Over time will gain strength and magic up to a max of +45 for both; is useful in longer battles.
Yuffie: If attacked physically while in the front row, she will counter with Mug. One enemy in the battle will have their given Gil doubled. After 15 turns have passed, Yuffie's Throwlette will charge and will throw a random shuriken weapon at an enemy when Defend is used (or if already Defending).
Cait Sith: Regains MP slowly over time; when Defending, the entire party will regain MP at a faster rate.
Vincent: Starts battle with Wall; will intermittently recover HP equal to his current MP.