@ricky73 in my mind you have the right approach.
Try the following to manually disable DHCP on a custom NAT configuration in Fusion 12:
answer VNET_2_DHCP yes
and change it to
answer VNET_2_DHCP no
You should find that there is no DHCP for that virtual network. To verify this, look in directory /Library/Preferences/VMware\ Fusion/vmnet2. There should be no file named dhcp.conf.
Now start your VM B without starting VM A that contains your DHCP server. It should not get an IP address until you start VM A.