The first thing you must do is determine exactly how much acetic acid you get from 25.0 g of vinegar.
Now, you could eyeball it and say that, because vinegar is 5% acetic acid, 25 grams of vinegar would have 4 times less acetic acid than 100 g of vinegar, which means you get roughly 1.25 g of acetic acid to work with (because 100 g of vinegar would have 5 g of acetic acid).
However, vinegar is 5% acetic acid by volume, which means you get 5 g of acetic acid per 100 mL of vinegar. Since vinegar has a density of 1.01 g/mL, that mass of vinegar would have a volume of
Using the 5% percent concentration by volume you know you have, that much volume of vinegar would contain
To determine how many molecules of acetic acid are present in that many grams, you need to figure out how many moles you have; this is done by using acetic acid's molar mass of 60.05 g/mol
Because 1 mole of any substance contains exactly
SIDE NOTE If you were supposed to use vinegar's 5% acetic acid concentration as being by mass, use the 1.25 g mass as a starting point to figure out the number of moles first, and then the number of molecules.