where to find the variables that can be changed through drush vset
Variables are saved in the variables table in the database. Every module can define its own variables. It is mostly common that the core and contributed modules save their configuration/settings as variables.

It is also mostly common that the modules add a prefix to their variables, so that you can look at the variables table in your database and search for the related module if they defined any variables.

Another approach would be looking in the source code of the configuration form for the related field. For example for file_temporary_path, you would see:

&lt;input type="text" class="form-text" maxlength="255" size="60" value="/tmp"
name="file_temporary_path" id="edit-file-temporary-path"&gt;


Here is the name attribute hints that this configuration field will be saved in file_temporary_path variable.

Moreover, if you have Devel module installed, there is a Variable Editor under the path devel/variable, which lists all the variables defined in your site and gives you the possibility to edit or delete them.