Hi ,
when you configure the scratch partition to a persistent all your doing is moving the logs location from RAM disk to a more Non-volatile disk, because we do not want to lose the logs after a reboot.
But it does not change the way how the logs are rolled over. there is a per-defined value for logs after which it gets rolled over, so that your storage does not get filled up only with log files.
If you wanna save all your logs change the log rotation options.
VMware KB: Controlling the size and number of vmkernel logs in ESX hosts
VMware vSphere 4 ESXi Installable and vCenter Server Documentation Center
Thanks,
Avinash