vifnum appears to be entirely a figment of network-route's imagination;
I can see no evidence of it elsewhere in the Debian Xen packages.
Since network-route claims to default to eth0 and does not document
vifnum at all, the obvious fix would be to change:
netdev=${netdev:-eth${vifnum}}
to:
netdev=${netdev:-eth0}
ttfn/rjk