I have two ISPs on my MX64 Internet connections. Each ISP assigns DHCP IP address on my MX.
Can I put any arbitrary IP address on the Virtual IP and it will work?
No. You need these to all be statics to work correctly. If your provider is giving you dynamics then you won't be able to set up a VIP on the WAN.
Besides what jdsilva suggests, the vIP also need to be a valid IP on the same scope.
Other than this, it doesn't really make any sense to configure a virtual IP address, if you do not have a Warm Spare, setup.
Actually I have my lab configured with HA. I was able to add the VIP but don't know what it adds on my recovery time.
I will test the fail-over time with and without the VIP and post the result here.