If you've been wondering how BRITNEY SPEARS' Las Vegas residency is going, here are a few updates ...

For starters, Britney is still lip-synching, which shouldn't come as a surprise.  There have been several videos from the show that supposedly "prove" she isn't singing live ... but the latest one is pretty great.

In the video, Britney misses her cue, and her vocals come back in while she's holding the microphone down at her side.  

Meanwhile, NICOLE RICHIE was pulled onstage at one of Britney's recent shows, and Britney hooked her up to a dominatrix-style LEASH.  Then, Nicole got down on all fours and Britney WALKED HER ... like a dog.

Britney later Tweeted, quote, "Can we all give a round of applause to @NicoleRichie for playing along in the show tonight?  You looked hot crawling on all fours ;)"

WARNING:  Turn down your speakers, the audio is HARSH.)