Miley Cyrus -- Crying in Vegas During First Post-Liam Performance

Miley Cyrus broke down crying in Vegas yesterday ... while she performed a song rumored to be about her break up with Liam Hemsworth.

Miley twerked it up on stage at the iHeartRadio Music Festival -- but the tears started flowing hard once she began singing "Wrecking Ball."

As we previously reported -- the couple has called it quits ... and Liam has gotten tight with telenovela actress Eiza Gonzales -- who BTW is crazy hot.

Despite the cry-fest ... Miley cleaned up for her second performance of the night.

And wore this.