@ricky73 in my mind you have the right approach.

Try the following to manually disable DHCP on a custom NAT configuration in Fusion 12:

• Make note of your custom NAT vmnet id. For example. I created a vmnet2 custom NAT configuration.
• Shut down Fusion.
• Edit the file /Library/Preferences/VMware\ Fusion/networking (you will need admin privileges to do so).
• You will find lines corresponding to your custom NAT configuration. Mine are prefaced by VNET_2 (that's what was included in my case for vmnet2).
• Change the line containing 

answer VNET_2_DHCP yes

and change it to

answer VNET_2_DHCP no

• Save and exit the editor
• Start Fusion.

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.