You should perform a full world and kernel install if on master.
Several people (including me) have been getting bit by a problem: when performing an installworld with a changed kernel, the vn kernel module is loaded, but it was built by the previous kernel and may cause problems when it doesn’t match up.
To fix that, vn is now built in, instead of being a separate module. The rescue initrd (which is what is being mounted when it has this problem) is now installed via a ‘make rescue‘ command that can wait until a successful installworld and reboot.