How to set default document root for if virtual document root doesnt exist?

I have set up virtual document root using vhost_alias and it works good if directory of subdomain exists. but it throws 403 error if subdomain dir doesnt not exist. I want to set a default dir if sub domain doesn't exist.

current vhost conf

<VirtualHost *:80>
        ServerAlias *.domain.com
        VirtualDocumentRoot /home/ubuntu/www/domain/%1
        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined

        <Directory "home/ubuntu/www/domain/*">
            Options Indexes FollowSymLinks
            AllowOverride All
            Order Allow,Deny
            Allow from all
            Require all granted
        </Directory>
</VirtualHost>

I tried using error document

ErrorDocument 403 http://app.domain.com%{REQUEST_URI}?%{QUERY_STRING}

but it changes browser url. Is there any way to redirect internally? i have also tried rewrite but that doesn't seems to work.

RewriteEngine On
RewriteCond %{DOCUMENT_ROOT} !-d
RewriteRule %{DOCUMENT_ROOT} "/home/ubuntu/www/domain/app"  [QSA,L]

tried this but not working

Found solution

RewriteEngine on
RewriteCond %{DOCUMENT_ROOT} !-d
RewriteRule (.*) http://app.domain.com/%{REQUEST_URI} [P,L]

Adding this inside Directory directive solved my problem. For SSL sites include this in Virutalhost block

SSLProxyEngine on
ProxyPassReverse / https://app.domain.com/