Netflow source-ip using VIP...

We're trying to export netflow from a Fortigate device to a collector behind a remote Fortigate using an IPSec VPN tunnel. To accomplish this in an environment where there could be overlapping subnets, we're using VIPs on the VPN interfaces. We can ping the remote collector by setting the ping source to a local VIP address. However when we try to configure the netflow's source-ip to a local VIP, it results in a "...not valid source ip." message.

Thanks in advance for any ideas.