Miley Cyrus Serenades 'Big Ole Ti***es' ... on Drag Queen

Well, she didn't motorboat 'em -- but Miley Cyrus did belt out a special rendition of "Happy Birthday" for a pair of boobs ... attached to a famous Hollywood drag queen.

Miley performed her boobtastic tribute during Beacher's Madhouse at the Roosevelt Hotel last night ... when she joined the celebration for Vicky Vox's birthday

Miley joined Vicky on stage, grabbed the mic, and sang ... "Happy birthday to Vicky Vox, with those big ole big titties ... happy birthday, Vicky Vox."