An easier way to do that would be with a default destination route and interface based track-ip monitoring. For example
set route 0.0.0.0/0 int eth0/2 gateway 184.69.6.121
set route 0.0.0.0/0 int eth0/3 gateway 68.179.20.150 preference 50
set int eth0/2 monitor track-ip
set int eth0/2 monitor track-ip ip 8.8.8.8 weight 100
set int eth0/2 monitor track-ip ip 4.2.2.2 weight 100
set int eth0/2 monitor track-ip ip 8.8.4.4 weight 100
This configuration would set interface eth0/2 down when all three of the above referenced IPs are unreachable (via ping). As soon as one of the above IPs is reachable, traffic would fail back to eth0/2. With this configuration, you would not have to manually intervien and you could set destination routes for the VPN tunnel.