Forum OpenACS Q&A: Re: Problem redirecting domain name to subsite (with apache virtualhost)
This would have the disadvantage that the use could then navigate above the subsite you want them in, into the old site's pages. However, I don't think there is a way around this.
If you want them to keep using Apache for whatever reason, set up a Virtual Host entry and use a combination of Redirect and ProxyPass to reach the subsite, e.g.
Redirect /index.html /my1stsubsite
ProxyPass / http://www.newdomain.nl:81/my1stsubsite/
ProxyPassReverse / http://www.newdomain.nl:81/my1stsubsite/
You will probably need to test to be sure everything works as you expect, especially on login, where OpenACS issues redirects itself.