I could add that you could modify the type of the emulated interface in Virtual Box settings and use the paravirtualized interface virtio, which will be seen in your NetBSD VM as virtio0. This allegedly should work faster, although I haven't gone through any benchmarks, but my VMs with this setup seem to be working fine.
I have only one problem presently with such a setup - when I use a bridged through wireless interface NetBSD VM and I connect it to a physical NetBSD machine attached to the same wireless network, I lose the connectivity between the NetBSD physical machine and the (Windows 10 in my case) Virtual Box host. I just have to ssh through another NetBSD machine connected to the wired interface of the same WiFi spot and from there to the other NetBSD machine and clear the arp cache.
Chavdar