An interesting point…it is possible for vfat.ko to be there even if the kernel build originally did not produce that file. For example, I sometimes add modules instead of building a whole new kernel…and the kernel’s original config.gz only reflects what it was originally built with. It seems one would have to check both the module directory “/lib/modules/$(uname -r)/” and “/proc/config.gz” to know for sure what is available.
In terms of the original question though it seems that the simplest solution is to build the vfat.ko module…if this module existed on the original system it would have loaded on attempt to mount unless there were some other odd error.