What actual hardware (NICs) do you have in this ESXi server? Is there anything showing in the logs that may give a hint of what the problem might be?
I generally prefer to configure the vlan on ESXi and not in the VM.Can you confirm you have the os-vmware (VMware tools) package installed in OPNsense?Do you know why it's showing 0.0.0.0 as one of the VM's IP addresses?
I'm using a Intel nic with 4 Ethernet ports.
What will be the difference off using a VLAN on ESXI instead of inside OPNsense?
It sounds like there was an IP address conflict on your network. Could it be that the Shield was using an IP address that was simultaneously in use by another device on the network?