The vSphere ESXi Dump Collector logs to different locations depending on the installation type:
VMware ESXi Dump Collector for Windows
When the vSphere ESXi Dump Collector service is installed on Windows, the logs are placed in:
%PROGRAMDATA%\VMware\VMware ESXi Dump Collector\logs\netdumper.log
VMware ESXi Dump Collector in the vCenter Virtual Appliance
The vCenter Server 5.x Virtual Appliance has a pre-configured Netdump service available. The logs are placed in:
/var/log/vmware/netdumper/netdumper.log