Bug 72333

Code at file /usr/src/sys/dev/vinum/vinumio.c , where vinum scans for drives looking for vinum partitions, it only considers slices existence (aka ad0s1h) when "__i386__" is defined. 

So, i suppose that code doesn't execute on amd64 machines and vinum doesn't find vinum partitions on a pc slice. So, it doesn't find vinum partitions (this is critical for mounting root filesystem over vinum) when you put vinum.autostart="YES" in /boot/loader.conf.

By hand, vinum works if you do "vinum read ad4", because it find the vinum partition at this drive. But "vinum read" doesn't scan on pc slices, too (i suppose that use the same scan routine than boot scan).

Fix: 

Probably code at file /usr/src/sys/dev/vinum/vinumio.c , where vinum scans for drives looking for vinum partitions, must consider slices existence (aka ad0s1h) when "__i386__" or "__amd64__" is defined (not only "__i386__".
How-To-Repeat: On a amd64 box: install with /stand/sysinstall over a slice (aka ad0s1). Configure a vinum partition for root on than slice (aka ad0s1h). 

Configure 
vinum_load="YES"
vinum.autostart="YES"
in /boot/loader.conf

Reboot (on single user, for example), and see "vinum: no drives found". 

Warning: test with vinum partitions only on slices (if vinum takes its configuration from another copy in any drive, i suppose it will read the configuration from that drive.