Firstly there is nothing wrong with acids. Vinegar is an acid. As is lemon juice. Both are very healthy.
Secondly emergen-C is buffered. In other words it is not acid but either neutral or even slightly alkaline.
Don't confuse the acid/alkalinity of a substance with its effect on the human body. Lemon juice and Apple-cider vinegar are both acids which have an alkalising effect on the body.