Is adding a large amount of Zamzam water to vinegar considered adulteration, like adding regular water to vinegar, or is it permissible to add Zamzam water to anything?

We do not know of any religious basis for the claim that mixing Zamzam water with any sold item is not considered deception. If the seller informs the buyer of the addition of Zamzam water or anything else to the mixture, then there is no problem with that. However, if the seller does not disclose this, and if the mixture is considered a defect that typically reduces the value of the sold item, then concealing it is religiously forbidden deception. A defect is anything that diminishes the value or essence of the sold item, frustrates a legitimate purpose, or is something that is customarily expected to be absent.