If you have the MySQL settings in the vBulletin config file correctly configured then it's not a problem with vBulletin, it's a problem with your MySQL server.
Either something happens that causes the MySQL server to crash or the MySQL's ini settings are not right.
Refer to:
http://stackoverflow.com/questions/5...-mysql-sock-38
http://alvinalexander.com/mysql/mysq...-server-socket < definitely look at this.