Let's call the time in minutes when the two vats have the same amount of liquid as "t".
After "t" minutes, the first vat would have a total of 6t gallons of liquid.
After "t" minutes, the second vat would have a total of 28 + 4t gallons of liquid.
To find the time when the two vats have the same amount of liquid, we can set up the equation:
6t = 28 + 4t.
Simplifying the equation, we can subtract 4t from both sides:
2t = 28.
Dividing both sides by 2, we get:
t = 14.
Therefore, the two vats will have the same amount of liquid after 14 minutes.