With City in the ascendancy looking for a third, there was a sting in the tail as Sweat felled Blessing to give Vela the opportunity to score his second from the spot in the 75th minute.
The Mexican DP did the rest, sending Johnson the wrong way to tie up the game at 2-2.
That was how it stayed, meaning City had to be content with a third draw from the opening three games of the season.
