According to the Online Etymological Dictionary, vacuum entered English in the 1540s directly from Latin as the substantivized, neuter form of the adjective vacuus. The earliest use was as an abstract, non-count noun denoting the emptiness of space, later any void or empty space, for which one could use the Latin plural vacua or simply tack on an s.
Centuries later, however, some were apparently dissatisfied with such an odd occurence of uu and tried out something that looked just as Latin, but more familiar:
A spelling vacumn, most likely in analogy to column and autumn, begins to appear in American newspapers in the latter half of the 19th century. Please note that not one usage occurs in the newspaper of a major city, but local papers from much smaller towns. I have found no dictionary, from that era or today, which accepts this spelling as an alternate.
…vacumn of coin… Ashland (OH) Union, 2 Aug. 1864.
In a perfect vacumn, that is, in a space free from air, the most perishable article will keep indefinitely. — Port Tobacco Times, and Charles County Advertiser (Port Tobacco MD), 17 Oct. 1873.
…produced the vacumn that caused the Mt. Carmel cyclone. — Wheeling (WVa) Daily Register,25 June 1877.
Dr. E. E. Buck would respectfully announce to the citizens of Canton and vicinity that he will open Remedial Parlors on Lawrence Avenue … on April 19th, for the cure of all Diseases, by the famous vacumn treatment. — The Stark County Democrat. (Canton, Ohio), 15 April 1880.
Mr. Villavaso … has charge of the vacumn pan from which superior sugars are made. — The Weekly Thibodaux (LA) Sentinel, 10 Dec. 1892.
If these reporters were consciously or unconsciously advocating for a new spelling of the word, their entreaties fell on deaf ears. The rest of the anglosphere, as in the centuries before them, retained the Latin spelling, even in the open compound that brought the word from the infinite void of space to more domestic settings: the vacuum cleaner.
Uses of the alternate spelling after the turn of the century inspire even less confidence:
A 1918 BS thesis from the University of Illinois:
A vacumn or inert atmosphere. — Frank Albert Martin, The Preparation of Metallic Barium, Thesis (B.S.) University of Illinois, 1918. Typescript.
Yet another newspaper article, this time from the 50s:
Tally said that the more he travels in the Seventh District "the more convinced I am that the voters will abolish the congressional vacumn that exists in this district when they go to the polls on Saturday, May 31." — Tabor City (NC) Tribune, 7 May 1952.
The transcribed remarks at a conference in Denver:
Who is really in charge of this outfit? There is no leadership, people are running around doing whatever in a vacumn. — National Applied Resource Sciences Center, Bureau of Land Management, “Report of the Rangelands Conference: held April 13-17, 1998, at the Double Tree Hotel, 3203 Quebec Street in Denver, Colorado.”
And, rather surprisingly, a technical report from NASA:
The electrostatic sector of the present system is formed from a single piece of machinable insulator that has desirable vacumn properties—low outgassing and ability to hold a vacumn. — NASA Technical Reports Server (NTRS), Machined Electrostatic Sector for Mass Spectrometer, 20 Feb. 2001.
Wherever you find the spelling vacumn, it in some ephemeral source like advertisements, newspaper articles, or other printed and digital writing that hasn’t undergone rigorous editing. To be accepted by a dictionary as an alternate, like catalog for catalogue, a spelling has to appear in enough newspapers of record, academic literature, government publications, fiction by established authors, etc., and perhaps adopted by educators and taught to others before it’s accepted. In other words, it doesn’t matter how many people on the internet spell it definately, that spelling will never be acceptable as standard.