Using an L2TP VPN to an private 10.x.x.x network results in a 10.0.0.0/8 route to this network ignoring other local tunnels. Why is that?
An L2TP connection is basically a PPP tunnel over an IPSec connection and the PPP protocol is so old that it predates the existence of subnet masks. When the PPP server at the remote end assigns you an IP address, PPP infers the subnet mask from the assigned IP address:
If the IP address starts with 192.168. the inferred mask is /24,
if the IP address starts with 172.16. to 172.31. the inferred mask is /16
if the IP address starts with 10. the inferred mask is /8.
Those where the old network classes and this is the official standardized behavior for the PPP protocol. For more information, please see
https://en.wikipedia.org/wiki/Classful_network#Classful_addressing_definition
If you don't like the entire 10.0.0.0/8 network to be routed over VPN, you need to ensure that VPN clients get an IP address from a different address range, for example an address from a 192.168.x.0 network (preferable with x not 0 or 1 as that will often collide with networks users have at home).
Any additional remote networks you can enter in VPN Tracker are optional and they are "additional" to the network you will gain access anyway derived from the assigned IP address which quite often is the only network required (in which case additional networks can be left blank).
A plain IPSec connection without PPP tunneling will not have this issue as IPSec is aware of subnet masks but when used for L2TP, IPSec only encrypts the PPP traffic whereas PPP handles the traffic routing.
