Since it is a one-time need (once they visit the URL they should always go to that subsite), you could just create another Virtual Host entry, and inside it put a Redirect directive.
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.
