Hello and thank you all for your help!
I´m not sure but maybe my VM/interface config is causing the problem.
All VMs in in vNet-B and vNet-C have the following configuration:
The VM in vNet-A only got one network interface with a public IP on it.
Could that configuration be the problem ?
To give you more insights: on eth1 there is also a vxlan-interface configured. (but i dont think that the vxlan is making any problem - it is just put on top - or isn´t it ?)