John Bean / Jean-Baptiste LeFebvre is my 3rd cousin through his mother. Many in my family had similar journeys from Quebec to the Champlain Valley, from the mid 17th century through the early 20th.
The evidence you seek is in the vital documents - look to the witnesses and godparents and you will find LeFebvre relatives. Fortunately, this work has been done by the inimitable Virginia DeMarce in her 1988 family reconstitution project for the Champlain Islands. "Notebook, French-Canadian Settlement in the Champlain Islands, Grand Isle County, Vermont Before the Year 1880"
By Virginia Easley DeMarce, Allen L. Stratton · 1988
https://sites.rootsweb.com/~vtgrandi/demarce/intro.htm
The parish repertoires for Vermont have been compiled by the Vermont French Canadian Genealogical Society and are available at the Vermont Genealogy Library.
https://www.VTGenLib.org
Edited to add: as far as date of immigration, the couple was married in 1836 at St Luc's in St Jean sur Richelieu, and their first child was born in Vermont in 1837. Subsequently, the rest of their children were born in Vermont. As there was much traveling back and forth by families in those days, this isn't necessarily proof of immigration in 1836, but it is an indication. Just keep in mind that it was not uncommon for children to be christened by a missionary in Vermont, brought home to Quebec, or both. There usually only ended up being one repertoire entry, but sometimes it did happen that there were two for the same child. The same thing happened with marriages, so it's possible they were already living in Vermont and went back to get officially married (having been married by a missionary or in another denomination besides Catholic).