At the same concentrations in solution, it takes more acetic acid for the same volume of citric acid to coagulate milk. The pKa value of acetic acid is 4.756, and citric acid is 3.128. Keep in mind that pKa is logarithmic. Lemons and limes typically have 5-8% citric acid, and household vinegar is 5%. So as a rule of thumb, you need to use 1.5x vinegar when a recipe calls for lemon juice.
If a recipe calls for citric acid powder (such as for mozzarella), then you need to calculate the equivalent volume for a 5% solution, and multiply by about 1.5 to get the same vinegar content.