Vidal spent two seasons in Spain at Barca, and helped them win the La Liga during his first season at the club.
Messi became close with Vidal during his two years at the club, and noted how his pal will be missed in the dressing room.
On his Instagram he said: “I only knew you from facing up against each other and you always seemed a phenomenon, but after I was lucky enough to get to know you personally and you surprised me even more.
“They were two years sharing a lot and you made yourself noticed a lot. The dressing room will miss you, Arturo.”
Luis Suarez also chimed in with a message for Vidal and said: “To have you as an opponent was unbearable because of how great you are as a player, but to have you as a team-mate, friend and to know you as a person, you are even greater!”
Vidal, 33, is set to be unveiled as an Inter player imminently as he was pictured in Milan over the weekend, and the club posted updates about him on its Twitter account.
It won’t be Vidal’s first time playing in the Serie A, where he previously had a stint at Juventus.
He will be re-uniting with manager Antonio Conte after the pair won three successive Serie A titles together at Juve.