が makes what is before be directly related to the verb/adjective. If it is a verb, then it tells us that what is before the particle is the performer of the action, if it is an adjective, then it tells us that it will be what is being qualified by the adjective. Example:
私が書いた手紙 : A letter that I wrote
女が好きな男 : A guy who likes girls
In your sentence, replacing が by で would change the sentence's meaning:
In this sentence, ビタミン could be seen as a cause/reason (unlikely) and that would make the sentence mean something like "because of the vitamins, we decided to...". It could be seen as a with (method) (unlikely) which would make the sentence mean "We decided to sell 多いしじみ etc. too, with vitamins". It could be seen as the 連用形 of だ (very unlikely) which would make the sentence mean "It is vitamins, and we decided to sell 多いしじみ as well".
None of these でs would make sense in this sentence.
In my opinion, here, で and が cannot mean the same thing, but it is kind of ambiguous because of 多い which is an adjective and not a verb. If it had a verb with it like in the phrase below:
I feel like it would be "more grammatically correct". And even if it was correct to use で with an adjective in this kind of sentence pattern, the fact that で can mean so many different things would make it too ambiguous for people to choose で over が.