Share with you guys my "working" upgrade approach from 6.0.2 to 6.1,
Hardware: HP Gen8
OS: ESXi 6.5
0) Download Jun loader 1.02a2 (DS3617xs)
1) Shutdown the VM, delete original IDE boot disk from setting (backup to somewhere if you like)
2) Copy synoboot (vmdk + img) to ESXi VM folder, add it as a new IDE drive (IDE 0:0)
3) Boot the VM, use assistant to find IP, install official pat
4) Wait for VM to boot up, access the vm console via telnet
(At this point seems some drivers are not being update with the DSM update script, nic won't be able to load in OS)
5) login to DSM, sudo -s as root user, execute "cp /lib/modules/update/* /lib/modules/"
6) reboot and done
I use this procedure to upgrade two of my DSM VM from 6.0.2 to 18.104.22.168, after upgrade the system runs for a week without issue.