This little tool, named SqlCmdVars.exe, lets you update you Database.sqlcmdvars file from the command line, dump its content, query the existence of variables, delete an existing variable, update the value of an existing variable and add new variable with value if required.
What is the scenario? If you are using vsdbcmd.exe to deploy your schema, you include the Database.sqlcmdvars, however vsdbcmd.exe does not have an option to override the values of the SQLCMD variables used at the command line level like you can do when using MSBuild (assuming you chained the SQLCMD variable to a MSBuild property).