Is what I swore by — that everyone who buys from vendors in front of mosques is a partner with them in sin — religiously sound?

If vendors disturb worshipers with their shouts and foul language, this is an reprehensible act that must be stopped, because the Prophet, peace and blessings be upon him, forbade disturbing a worshiper by raising one's voice. Selling in the mosque is disliked (makrooh), and if vendors spread out their goods in the mosque's precincts (hareem) and constrict the worshipers, their action is forbidden to them. As for your oath regarding the sinfulness of those who participate with them, it is a mistake, and you are not required to make expiation for it. We advise you to offer gentle counsel and cooperate with the worshipers to prevent them.