The ATTACH documentation doesn't say it, but attached databases use the same VFS as the main database unless explicitly specified. This is especially surprising with memdb as it accepts any name.

Here is an example showing it happening.

sqlite> .open 'file:/shared?vfs=memdb'
sqlite> attach diskdb as diskdb;`
sqlite> .databases
main: "" r/w
diskdb: "" r/w
sqlite> detach diskdb;
sqlite> attach 'file:diskdb?vfs=unix' as diskdb;
sqlite> .databases
main: "" r/w
diskdb: /space/apsw/sqlite3/diskdb r/w