He's left with immediate effect and will provide backline cover.
Speaking at Kingsholm this afternoon, Vaughan spoke on behalf of everyone at the club when he expressed his delight for Twelvetrees.
"We're all very, very proud of Billy. I think, first and foremost, it's a huge endorsement of the way Nigel (Davies) has developed the squad. We've got some great young talent coming through.
"And it's obviously credit to Billy as well. He was at Leicester, it was all that he knew and it's the mark of the man that he stepped out of his comfort zone and he came to Gloucester to challenge himself and develop his game.
"He's played incredibly well for Gloucester and for England. It's a fine achievement for Billy and a proud moment for him, his family and everyone at Gloucester Rugby."
A call up of this kind is significant, and Vaughan confirmed that it reinforces Gloucester's place on the world rugby map.
"The Lions brand is world renowned and we're delighted to have a player in the squad. I think Phil Vickery was our last representative, back in 2001 in Australia.
"It's brilliant for everyone connected with the club. Already, the messageboards, Facebook and Twitter are buzzing with messages.
"It's great for Gloucester as the gaze of the rugby world is back on us for a while, especially with Rugby World Cup 2015 around the corner.
"I really believe that Billy, if he gets some game time over there, will do himself and Gloucester Rugby proud."