The drama is unfolding on the set of the "Vanderpump Rules" reunion show ... the cast members arrived around 9:45 AM ahead of the highly-anticipated taping.

Tom Sandoval showed up to the San Fernando Valley set looking like he's going to a funeral -- dressed in all black -- which may be fitting, because he's likely to be buried for cheating on Ariana Madix with fellow cast member Raquel Leviss.

Ariana, fresh off her recent brand partnership with a sex toy company, is glowing ... she's rocking a yellow hoodie and matching sweats, and she just got her nails and hair done.

She definitely is sending Tom a signal ... her head-to-toe ensemble by Boys Lie includes graphics that read, "WE'RE SORRY THE NUMBER YOU ARE TRYING TO REACH HAS MOVED ON."

Lala Kent pulled up with a huge designer bag ... and we've also seen arrivals from Katie Maloney, Scheana Shay and Tom Schwartz.

Of course, it wouldn't be a "Vanderpump Rules" reunion taping without the big dogs ... Lisa Vanderpump's in the house, and she was met with a huge embrace from reunion moderator Andy Cohen.

No sign of Raquel yet ... remember, her restraining order against Scheana is complicating the reunion ... but she is in L.A., because we got her yesterday at LAX and a nail salon, and she was pretty chatty.

As we first told you, Raquel and Scheana have to be kept apart when they're both on set. When one of the women is filming with the rest of the cast, the other has to be sequestered in a hair and makeup trailer.

The seating chart is out too, and there are two versions. When Raquel is filming, she's sitting between the Toms and is five seats away from Ariana. When Scheana is on the set and Raquel's in her trailer, Scheana is seated at the end after Tom Schwartz.