Autosync master and slave when master goes offline
No.

Replication will survive the VPN going down, but a crash of the master host does not reliably flush the master's binary log, so on reboot there is no guarantee that the master's Binlog Dump thread does not send the same events to the slave's I/O thread.

You should find a solution that involves the master server staying online. Failing that, you might get away with having the slave run for short periods during which the master is less likely to crash, so that you reduce the possibility of the server crashing during log transfer, but it is not a reliable solution.