у мя тож когда-то такая проблемка была.
так вот что я сделал: просто добавил в конфиг идейца среди вирт. хостов еще и основной. как то через жопу, зато работало.

Пробывал, не работает(

мм.. приведи полный конфиг апача, что ли..

Listen 80
NameVirtualHost xxx.xxx.xxx.xxx:80
<VirtualHost forum.site.ru:80>
<Directory "/forum">

Options None
AllowOverride all
Order allow,deny
Allow from all
</Directory>
DocumentRoot /var/www/forum
ServerAlias forum.site.ru www.forum.site.ru
ServerAdmin webmaster@example.com
DocumentRoot /forum
ServerName forum.site.ru
</VirtualHost>

<VirtualHost xxx.xxx.xxx.xxx:80>
DocumentRoot /htdocs
ServerName www.site.ru
<Directory "/htdocs">
Options None
AllowOverride all
Order allow,deny
Allow from all
</Directory>
</VirtualHost>

<Directory "/htdocs">
Options None
AllowOverride all
Order allow,deny
Allow from all
</Directory>

DocumentRoot /var/www/htdocs/
#ServerName site.ru

ServerType standalone
ServerTokens Prod
ServerRoot "/var/www"
LoadModule php5_module /usr/local/lib/php/libphp5.so
Addtype application/x-httpd-php .php .php3


PidFile logs/httpd.pid
ScoreBoardFile logs/apache_runtime_status
#ResourceConfig conf/srm.conf
#AccessConfig conf/access.conf
Timeout 40
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 15
MinSpareServers 5
MaxSpareServers 10
StartServers 5
MaxClients 50
MaxRequestsPerChild 0

MaxCPUPerChild 0
MaxDATAPerChild 0
MaxNOFILEPerChild 0
MaxRSSPerChild 0
MaxSTACKPerChild 0

Include /var/www/conf/modules/*.conf

ExtendedStatus Off
Port 80

User www
Group www

#ServerAdmin usersoff@site.ru
#ServerName site.ru
SSLEngine Off
# DocumentRoot "/var/www/htdocs"
<Directory />
Options none
AllowOverride None
Order deny,allow
Deny from all
</Directory>

#<Directory "/var/www/htdocs">
# Options none
# AllowOverride None
# Order allow,deny
# Allow from all
#</Directory>
UserDir disabled

DirectoryIndex index.php index.html

AccessFileName .htaccess
<Files .htaccess>
Order allow,deny
Deny from all
</Files>

UseCanonicalName On
TypesConfig conf/mime.types

DefaultType text/plain

<IfModule mod_mime_magic.c>
MIMEMagicFile conf/magic
</IfModule>

HostnameLookups Off

#ErrorLog syslog:daemon
ErrorLog logs/error_http_log
LogLevel warn

LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
LogFormat "%h %l %u %t \"%r\" %>s %b" common
LogFormat "%{Referer}i -> %U" referer
LogFormat "%{User-agent}i" agent

CustomLog logs/access_log common

CustomLog logs/referer_log referer
CustomLog logs/agent_log agent

#CustomLog logs/access_log combined

ServerSignature Off

ScriptAlias /cgi-bin/ "/var/www/cgi-bin/"

<Directory "/var/www/cgi-bin">
AllowOverride None
Options None
Order allow,deny
Allow from all
</Directory>

IndexOptions FancyIndexing

IndexIgnore .??* *~ *# HEADER* README* RCS CVS *,v *,t

AddEncoding x-compress Z
AddEncoding x-gzip gz

AddLanguage en .en
AddLanguage fr .fr
AddLanguage de .de
AddLanguage da .da
AddLanguage el .el
AddLanguage it .it

LanguagePriority en fr de

BrowserMatch "Mozilla/2" nokeepalive
BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
BrowserMatch "RealPlayer 4\.0" force-response-1.0
BrowserMatch "Java/1\.0" force-response-1.0
BrowserMatch "JDK/1\.0" force-response-1.0
<IfDefine SSL>
AddType application/x-x509-ca-cert .crt
AddType application/x-pkcs7-crl .crl
</IfDefine>

<IfModule mod_ssl.c>
SSLPassPhraseDialog builtin
SSLSessionCache dbm:logs/ssl_scache
SSLSessionCacheTimeout 300
SSLMutex sem
SSLRandomSeed startup builtin
SSLRandomSeed connect builtin
#SSLRandomSeed startup file:/dev/random 512
#SSLRandomSeed startup file:/dev/urandom 512
#SSLRandomSeed connect file:/dev/random 512
#SSLRandomSeed connect file:/dev/urandom 512
SSLRandomSeed startup file:/dev/arandom 512

SSLLog logs/ssl_engine_log
SSLLogLevel info

</IfModule>

Таке чуство что forum.site.ru тоже самое что site.ru...

Так не пробывал?

Apache 2.2:
А вообще на офф сайте в доках все есть!