I just finished my indy playthrough and got Arcade to fight with the remnants for the NCR. I saw Moreno, Henry, Kreger and Johnson leave the vertibird but saw no sign of Arcade. Where was he? Was it a bug?

You can find him in the dam's main reception room.

That's an....interesting place for him to be.
I was hoping he would get out of the vertibird and follow me to the Legate, zapping Legionaries left, right and center, just like old times.

IMO the remnants appearance at Hoover Dam could have been done better. The vertibird just sort of spawns on the dam rather than coming in like Bear force one.
Then Moreno just stands there, Kreger (if you persuade him to help) just walks away, Dr. Henry only goes to the end of the dam and Arcade is back at the place where you start the finale.