Let \( t \) be the number of minutes after the filling begins.
For the first vat, which starts empty and fills at a rate of 6 gallons per minute, the amount of liquid in the vat after \( t \) minutes can be expressed as:
\[ \text{Amount in first vat} = 6t \]
For the second vat, which already contains 28 gallons and fills at a rate of 4 gallons per minute, the amount of liquid in the vat after \( t \) minutes can be represented as:
\[ \text{Amount in second vat} = 28 + 4t \]
To find out when both vats will contain the same amount of liquid, we set the two expressions equal to each other:
\[ 6t = 28 + 4t \]
Now, we will solve for \( t \):
- Subtract \( 4t \) from both sides:
\[ 6t - 4t = 28 \]
\[ 2t = 28 \]
- Divide both sides by 2:
\[ t = 14 \]
Thus, the two vats will have the same amount of liquid after \( \boxed{14} \) minutes.