ip rule add table 128 from 50.1.2.3
ip route add table 128 to 50.1.2.0/24 dev tun0
ip route add table 128 default via 10.10.10.5

root@server:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.10.10.1      10.10.10.5      255.255.255.255 UGH   0      0        0 tun0
10.10.10.5      0.0.0.0         255.255.255.255 UH    0      0        0 tun0
0.0.0.0         10.10.10.5      128.0.0.0       UG    0      0        0 tun0
128.0.0.0       10.10.10.5      128.0.0.0       UG    0      0        0 tun0
0.0.0.0         0.0.0.0         0.0.0.0         U     0      0        0 venet0