City have signed 16 year-old Vickery for around £200,000.
Mackay said: “Ben is a young player with a lot of potential, which is why there was a lot of interest in him from elite clubs below the border. Developing young players with this potential is credit to our staff and the Hibernian FC Academy.
“When clubs like Manchester City show interest in one of our young players, understandably it becomes an opportunity that’s difficult for them to turn down.
“After Ben’s successful trial, we worked hard with Manchester City to get a deal that represented his true value rather than the player running his contract down and Hibs receiving the usual training compensation.
We’d like to thank Manchester City for how they conducted themselves during the negotiations, and wish Ben all the best for the future.”