Just a quicky question:
While I'm running tails in a VM. (with NIC on NAT)
Is it safe to use the internet on my host computer (not connected to tor-network) of course.
Or is there like a route to the VM which causes a potential attacker to intercept all data?
Please reply.

No. The host OS' traffic will be routed the same no matter if Tails is run in a VM or not.