Hi, may i know how to set vendor default logo, instead of showing demo store.
It appears it is based on the default logo for the theme being used.
Look at the file design/themes/[YOUR THEME]/manifest.json and you'll see where the 'logo' is set. Same for the 'mail' logo and 'favicon'.
You can create your own manifest.json and the use the hook 'styles_get_manifest' retrieve it. This hook would then retrieve your version of the manifest.json instead of the default distributed with the theme. But make sure you compare the mainifest.json after every upgrade to ensure that other changes (extensions) have not been made.
Make sure you clear the template cache (?ctpl) and the registry cache (?cc) if you implement the hook.