If a spell is cast with an alternative cost that includes mana (such as flashback), Vexing Bauble's first ability won't trigger and won't counter that spell. Similarly, if a spell is cast without paying its mana cost, but the player did pay mana for additional costs such as from kicker or from Thorn of Amethyst, Vexing Bauble's first ability won't trigger and won't counter that spell.