VitalPBX uses Asterisk as it’s telephony engine. Asterisk uses it’s in memory DB, (IIRC, SQLite) and then there’s MySQL which Asterisk uses to log CDR and CEL events, as well as what VitalPBX uses to read/write it’s configuration.
The VitalPBX installer assumes that MySQL runs on localhost (127.0.0.1) while remote DBs are definitely doable, it doesn’t seem like it is supported right now.
Out of curiosity, why would you use a remote DB instead of one on the same VM?