The phrase "speak vainly" can be replaced with many synonyms to convey the same meaning. One of the simplest synonyms is "speak fruitlessly," which indicates that the speaker's efforts are in vain and will not yield the desired results. Other more expressive synonyms could be "blather," "prattle," or "gab," which all denote excessive talking without purpose. "Ramble on" and "babble" are two additional synonyms that suggest a lack of focus or direction in one's speech. Overall, there are many alternatives to the phrase "speak vainly," depending on the context and tone of the statement, that allow writers and speakers to vary their language and convey their intended meaning.