You will need an interface ip address on the Juniper that is included across the vpn to reach the syslog server.
Once you have a valid address you can test this using ping sourced from that interface
ping 1.1.1.1 interface ge-0/0/0.0
and then also configure your syslog stanza to use that ip address as the source
set system syslog source-address 2.2.2.2