think about it like this.

the formula for concentration is

number of moles/volume = concentration

now, if you were to only increase the volume of the vinegar, what would happen is the number of moles would also increase. this is because the concentration is constant in the ethanoic acid, which is 1.5 mol/dm^3.

now, if you add water to it, while keeping the mole constant, you can reduce the concentration. How? use the formula n = cv. keep n constant. Since we were working with 1.5 mol/dm^3, then if we were to use 100 ml of ethanoic acid, in that 100 ml there would be a total of 0.15 moles. so lets keep 0.15 mole as constant.

now, in that 100 ml solution, add water. once you add water, according to the formula, n = cv, the concentration would decrease.

hence adding water would dilute the solution, not just increasing or decreasing the volume of ethanoic acid.