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Maybe I'm not following your idea, but what's wrong with the sentence "Я куплю что-нибудь наверное, когда я поеду в книжный магазин"?
Когда feels perfect here.

The correct ways with "во":

Поездка во Францию.
Нет смысла во вражде.

So, "o" is added when the rhythmics of speech is needed. Otherwise, it's incovenient to pronounce. Also, it's incovenient...