Well, baking soda is sodium bicarbonate (NaHCO3) with a molar mass of 84.007 g/mol. Vinegar is acetic acid (CH3COOH) with a molar mass of 60.05 g/mol. Each molecule of sodium bicarbonate give one molecule of CO2 with a molar mass of 44.0095 g/mol and density of 1.98 g/L as a gas.I don't think Vinegar and baking soda in small quantities are enough to blow up a balloon. I was thinking more along the lines of dry ice + water, but don't know if that will work. and dry ice itself is too cold anyway. the ideal solution would be a fairly slow chemical reaction that start with a bit of reactants and end up producing lots of gas.
I wonder what the shelf life would be for this propellant?I'd go with just dry ice, really. You need so little that you probably won't need to worry about either the cold, or use water to get it to sublimate faster.