I guess you are trying to do backup under vServer to the arbitrarily folder on vServer side. It is not allowed, you can do so only in the "local" mode. vServer's backups are placed into SYSTEMBACKUPCATALOG which is defined in the ini file (Also you may see/change that setting via SQL - "SHOW PROPERTIES OF SERVER"/ "GET/SET PROPERTY .... OF SERVER ..." ).
So, you can do only "BACKUP DATABASE" under vServer.
One more thing - currently Valentina does not support "network file-path notation" like "\\server\share\...". You should map it to make it available as a regular path like "F:\..."