Apache proxypass css/js to their specific URLs

I have two of my backend sockets running at port 9000 and 9001. Both provide a WebUI with CSS,JS and PHP. But both are different websites and when I use proxypass for both sites, how can I load css,js from the requested URL itself?

<VirtualHost *:80>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI}
</VirtualHost>

<VirtualHost *:443>
        ServerAdmin [email protected]
        ProxyRequests off
        DocumentRoot /var/www/html
        SSLProxyEngine on
        ProxyPreserveHost On
        SSLProxyVerify none
        SSLProxyCheckPeerCN off
        SSLProxyCheckPeerName off
        SSLProxyCheckPeerExpire off
#       ServerName _

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined
# Possible values include: debug, info, notice, warn, error, crit, alert, emerg.
        LogLevel error

        SSLEngine on
        SSLCertificateFile /path/to/crt
        SSLCertificateKeyFile /path/to/key


        RewriteEngine on
        RewriteRule ^/site1$ /site1/ [R]

        ProxyPass /site1/  https://127.0.0.1:9000/
        ProxyPass /vendor/  https://127.0.0.1:9000/vendor
        ProxyPass /css/  https://127.0.0.1:9000/css
        ProxyRequests off
        ProxyHTMLURLMap https://127.0.0.1:9000/  /site1/
        ProxyHTMLExtended On
        <Location /site1/>
            ProxyPassReverse /
            ProxyHTMLEnable On
            ProxyPreserveHost On
            ProxyHTMLURLMap  /  /site1/
            RequestHeader    unset  Accept-Encoding
        </Location>

        RewriteRule ^/site2$ /site2/ [R]

        ProxyPass /site2/  https://127.0.0.1:9001/
        ProxyPass /vendor/  https://127.0.0.1:9001/vendor
        ProxyPass /css/  https://127.0.0.1:9001/css
        ProxyRequests off
        ProxyHTMLURLMap https://127.0.0.1:9001/  /site2/
        ProxyHTMLExtended On
        <Location /site2/>
            ProxyPassReverse /
            ProxyHTMLEnable On
            ProxyPreserveHost On
            ProxyHTMLURLMap  /  /site2/
            RequestHeader    unset  Accept-Encoding
        </Location>

</VirtualHost>

As you can see above, I have added

ProxyPass /vendor/  https://127.0.0.1:9000/vendor
ProxyPass /css/  https://127.0.0.1:9000/css

both these lines to manually pass the CSS,vendor files to the proxy URL. But how can I make the same to site2? Since /css is added above, in site2 am getting css content from site1 itself. Also to make all hyperlinks in the site have the /site1/ and /site2/ path, I have added ProxyHTMLURLMap / /site2/ line in both of them. Its working. But no sure why CSS, JS files still go to / root path since root path doesn't have any files and give 404. How can I make these internal call CSS,JS files also take prefix /site1/ or /site2/

Without any of the ProxyHTMLURLMap / /site2/ all the links give 404 since requests go for / instead of /site2/ Same thing I want to do for CSS/JS files as well.