Yes, without proper knowledge, viruses can escape from VM not only to your network, but also your host.
I think you already know there's some sharing settings between host and VM, be careful about that.
About your concern that they infect other computer, you can either set your VM guest to "internal network only", or make sure all devices are secured in your LAN.
MERGED QUESTION Question from Pootislord : "Can a virus escape a virtual machine through the internet?"
So I plan on using a virutal machine (VMWare) to mess arround with viruses like BonziBuddy. Now, viruses cant escape the virtual machine onto the disk, but could they affect computers on my network? If so, should I use a VPN or will that not help?