If it is working on with one of Friendly URL setting it is good news - you have no issues with vBET
So - when you change Friendly URL to Mod rewrite you have error - it means that issue is in .htaccess file just as you suspect.
First - please make sure that your .htaccess file is OK for normal (not translated) URLs. So set there rules required by vB and test it. After that put vBET rules BEFORE vB rules. This is how looks .htaccess file on our test vB4 forum:
Code:
RewriteEngine On
RewriteRule ^/?(af|sq|ar|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|de|el|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|vi|sq|ca|et|tl|gl|hi|id|lv|lt|mt|sl|uk|vi|cy|yi)/archive/index.php/(.*)?$ archive/index.php?/$2&language=$1 [L,QSA]
RewriteRule ^/?(af|sq|ar|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|de|el|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|vi|cy|yi)/$ index.php?language=$1 [L,QSA]
RewriteRule ^/?(af|sq|ar|be|bg|ca|zh-CN|hr|cs|da|nl|en|et|tl|fi|fr|gl|de|el|iw|hi|hu|is|id|ga|it|ja|ko|lv|lt|mk|ms|mt|no|fa|pl|pt|ro|ru|sr|sk|sl|es|sw|sv|zh-TW|th|tr|uk|vi|cy|yi)/(.*)?$ $2?language=$1 [QSA]
#vB FRIENDLY URL RULES
RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^threads/.* showthread.php [QSA]
RewriteRule ^forums/.* forumdisplay.php [QSA]
RewriteRule ^members/.* member.php [QSA]
RewriteRule ^blogs/.* blog.php [QSA]
ReWriteRule ^entries/.* entry.php [QSA]
RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^(?:(.*?)(?:/|$))(.*|$)$ $1.php?r=$2 [QSA]