You are correct, and that is exactly why I don't drink diet drinks or eat diet food, with artificial sweeteners.
"The dose makes the poison" and the minuscule amount in an oral vaccine, (that is not forced on anyone), compared to the amount of this particular poison in food and drinks, among other uses, is insignificant.
Again, it is up to the individual to decide whether they wish to have the vaccine, or not. People are very capable of making up their own minds, given all the information.
Personally, I would rather have the oral vaccine, even if it contained aspartame, then increase the risk of contracting typhoid, if I were to be in a circumstance that made this likely.