HELP!! Redirecting domain URLs

Hi. I have a vbulletin board running with vbseo, with a vbadvance portal.
It has 2 active domains (www.domainname.net/forum, and www.domainname.com.ar/forum). The latter is currently a secondary domain and we would like to redirect all the traffic to the .net one.

Doing the recommendations i have read in this forum i was able to actually make it work with a 301 redirect, but ONLY for the main url (Domainname.com.ar ----> www.domainname.net). When it comes to redirecting www.domainname.com.ar/forum to www.domainname.net/forum, it just doesn't work.

I think the problem has to do with the fact that our board is in the /forum directory, and in / we got the vbadvance portal.

Can anyone help me out with this?

Thanks in advance!

What is the .htaccess inside of the forum root on your .ar?