I've found from playing around with this on ESXi 6.5 that I've had the most success when I set my VM to use EFI and not BIOS. EFI is in the VMX supplied with Jun's loader zip, but if you are creating your own VM then probably haven't set specifically to EFI.
I have the 50mb synoboot, an 8GB "system" drive VMDK on SSD, 2x 750GB HDs passed thru to the VM as RDMs, and 2x USB3 drives passed thru via ESXi (not Direct I/O)
Added a 2nd 8GB disk as VMDK on SSD and added this as SSD Cache in DSM.
Network-wise, I added 2x VMXNET3 NICs and bonded them in DSM
Love the way I can mount Windows shares on the Syno, and then share them out via NFS/AFP so it acts as a gateway.
Loving this software