I get the same behavior regardless of if I try to mount with udp or tcp.
You could also switch to TCP mounts, instead of UDP (which I'm just
assuming you are currently using.)
Fastpath should be the answer, but with fastpath off, UDP packets that
were sent to VIF1, that are replied to by VIF2 (due to the filers
route table), will have a source address of VIF2 (and hence be dropped
by the mounting system.) TCP packets will be source with the IP
address of the destination interface on the filer, regardless of the
egress interface.
So switching to TCP mounts will make your current config work rapidly,
but not necessarily with the transit paths as you want.
(The above is characteristic of UDP - not just NetApps.)