You can definitely uses plain old white vinegar. It too is a weak acid (acetic acid). All you need is a source of hydrogen ions (H+) which are hydrogen atoms that have lost an electron.
For the mildly curious:
It doesn't matter what the source of H+ is, only that there are H+. The hydrogen ions from hydrochloric acid (a strong acid) are identical to the hydrogen ions from citric acid (a weak acid). For our purposes, all a weak acid means is that it is safe to handle. We don't need to worry about how many H+ ions are produced per amount of acid, there is going to be plenty of them for our purposes.
Hydrogen ions can be generated via electrolysis (electricity) from plain old water. It involves breaking the hydrogen bonds that hold the water molecule together. This takes energy. Not very efficient especially at 12 volts. It would take a long time to generate enough hydrogen to make this work.
Adding an acid to water results in the formation of H+ (and a negatively charged anion that depends on the acid used, but makes no difference since all we care about are the H+ ions). What you end up with is a bunch of H+ ions floating around. The water molecules do all the work and an external energy source isn't needed.
So why the battery?
So now you have a bunch of H+ ions floating around which are all revved up with no place to go. Now we turn on our battery. Instead of cleaving water molecules, we are now attracting the H+ ions in the soup to the negative terminal (opposites attract). It's here at the negative terminal that the magic happens.
As the H+ ions are being attracted to the negative terminal, the negative terminal is shedding electrons (electrons have a negative charge). The H+ ion picks up one of these electrons and becomes a hydrogen atom (no charge and no longer attracted to the negative terminal). There is still one more step. Each hydrogen atom being produced bonds with a second hydrogen atom to form hydrogen gas - hence the 2 in H2. This then bubbles out of solution and collects in the assembly the rocket sits on waiting to be ignited. BTW you probably noticed water drops after the hydrogen is ignited. The reaction results in hydrogen reacting with the oxygen in the air forming H2O.
So how much vinegar do you need? You could determine the ration of citric acid to vinegar needed to produce the same number of H+, but that would be too much work.
I'd start with distilled water in the generator and add vinegar and turn it on to see how many bubbles are produced. Repeat until you get a nice stream of hydrogen gas bubbles.
Or just add 9 grams of citric acid crystals and be done with all this nonsense once and for all.