Assuming you have a default outbound source nat policy, you want to add something like this to the nat rule to exclude the VPN traffic.
set security nat source rule-set trust-to-untrust rule vpn match source-address 10.0.100.0/24 set security nat source rule-set trust-to-untrust rule vpn match destination-address 192.168.178.0/24 set security nat source rule-set trust-to-untrust rule vpn then source-nat off insert security nat source rule-set trust-to-untrust rule vpn before rule source-nat-rule