Calculate the percent of acetic acid in the vinegar. (the density of vinegar is 1.002 g/ml.)

% acidity = (grams of acetic acid / grams of vinegar) x 100

The other method you can use is to remember that in order to calculate the percent acetic acid in the vinegar solution by mass you can assume the density of the vinegar solution is the same as water (1.00 g/ml).  The formula will be:

grams of vinegar solution = density of vinegar solution x volume of vinegar solution

For example if 5 ml of the vinegar solution was used and if we assume that  density of the vinegar solution is 1.00 g/ml then 

grams of vinegar solution =  1.00 g / ml x 5 ml  = 5 grams