VFSes just a c object created as part of the sqlite runtime and are registered and activated ^1 with the c api. You can recompile the sqlite command line to open with a given vfs object, or enable the URI filenames feature ^2 which allows opening a file with a given vfs. For the appendvfs in th original post, you probably need to use a new version of sqlite with it in the sourcecode

^1 https://www.sqlite.org/c3ref/vfs_find.html

^2 https://www.sqlite.org/uri.html