'vehemently assert' is correct and usable in written English.
You can use it when you want to say that someone or something is making a strong point, usually by being emphatic or passionate about it. Example: She vehemently asserted that the new policy was unjust.
Fans of Mr. Chou vehemently assert that his music is hip-hop, while denigrating groups like Yin Tsar.
Unlike the others it walks softly and has little or no shtick, affirming the voice of the artist as much as Venice and Documenta vehemently assert the curator's sovereignty.
There's a presumption of innocence and they vehemently assert their innocence," Miles Feinstein, an attorney for Joe Giudice, told reporters before the arraignment.
A coalition of community leaders vehemently asserted that the four defendants were wrongly accused and targeted because they were black.
The party now vehemently asserts that Uighur terrorism is directed by Islamist militants based abroad and is increasingly rooted in extremist ideas picked up on the internet.
"For those who found him charming, brilliant and sincerely devoted to the welfare of his country there were others who vehemently asserted that he was calculating, overly ambitious and ruthless".
By Andy Borowitz September 1, 2016 MEXICO CITY (The Borowitz Report) — The war of words between Donald J. Trump and Mexican President Enrique Peña Nieto continued on Thursday as Peña Nieto vehemently asserted that he made the Republican Presidential nominee pay for lunch during his visit to Mexico City.
He vehemently asserted that ranch hands and their families were free to come and go, after the Larsens and other ranchers were faced with government claims that ranches in their region held their Guaraní workers in servitude; the government has used the charge to move ahead with land seizures.
It is the aggressive attempt to export liberal permissiveness that causes fundamentalism to fight back vehemently and assert itself.
Here the musicians went so far out on so many limbs that at one point in the middle of the Brahms C minor Quartet, which concluded the evening, you had an image of the four players hanging by their arms from the thin branches of four separate trees, each continuing vehemently to assert his or her own voice in a compelling discussion among equals.
Although no direct evidence exists that Crane formulated a precise theory of his craft, he vehemently rejected sentimentality, asserting that "a story should be logical in its action and faithful to character.
