I configure a VTI interface with 'ip link add vti0 type vti local aa.bb.cc.dd remote aa.bb.cc.de'

# bird --version
BIRD version 1.6.4

# ifconfig vti0
vti0      Link encap:UNSPEC  HWaddr C0-A8-03-03-00-00-00-00-00-00-00-00-00-00-00-00 
          inet6 addr: fe80::5efe:c0a8:303/64 Scope:Link
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1364  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:3 errors:1 dropped:0 overruns:0 carrier:1
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:252 (252.0 B)

I configure bird with a static device route through that interface:
...
protocol device {
  scan time 10;           # Scan interfaces every 10 seconds
}
protocol direct testD {
  interface "*";
}
protocol static testS{
  route 10.20.0.0/24 via "vti0";
}
...