I've had that situation before where I couldn't mount the partition that vi was on. What I did was use ed to edit /etc/fstab. Yeah, it's a pain in the rear, but if you have a print out of the basic ed commands next to you, you can limp your way through it.
By the way, you know that you don't have to do mount -a to mount specific partitions, right? You could just mount a single partition with mount /dev/ad0s1d /usr, or whatever it happens to be...