Sure vbet is your problem? Other plugins all disabled for test?

yes. The site works fine until i install vbet.

What version of PHP are you running? VB 4.2.5 will work with and up to PHP 7.1 (might limp along under PHP 7.2 with warnings turned off). Mods are completely hit and miss. Remember the majority were written when PHP 5.x was the norm. vBet is clearly the issue and you will need to work with a PHP developer (you do not need a Vbulletin developer) to look at your PHP log file and recode the lines that have issues. It really should not be too difficult...