Gzip compression has little to no performance impact on server operations. You can leave it on. It only requires cpu cycles which most servers have plenty to spare.
The increased load is caused by the mysql queries on the vbulletin setup when traffic exceed IO capacity.
It looks like your problem just sprung up out of no-where.
1) Did you make any changes that led to this problem? Looks like your PHP backend is having issues.
2) Are you using the latest PHP LSAPI, LiteSpeed? Just making sure.
3) It's strange that the vbulletin setup can read fine but post operations return errors.
The most important question is 1. Since something changed that caused this problem to happen. Finding out what changed will reveal the source of the problem. Increase in traffic should not cause this problem. Test your php binary by going "/lswsinstall/fcgi-bin/lsphp -i" to see if you get any errors from the command or in the "/lswsinstall/logs" log files.