I have the same issue here. The CIFS components of (Raspi) OSMC do not support the “vers” option of CIFS mount. The generic Raspbian from the foundation does. That the module has version has 2.09 does not mean that CIFS 2.0 is supported and the version of CIFS supported with the “vers” option is 3.
So it would be very helpful if the “vers” option would be supported as SMB1 should not be used anymore, e.g. for security and performance reasons.