Log into your vSphere host using ssh as root.
Assuming 608ee632-9b6d36e6-0286-b47af13db000 is the datastore you want to use:
cd /vmfs/volumes/608ee632-9b6d36e6-0286-b47af13db000
mkdir log
Or use any other directory name you like.
esxcli system syslog config get
Will show you the current log location, which, if you were came here from a search engine, is probably []/scratch/log and refuses to be changed.
esxcli system syslog config set —logdir=/vmfs/volumes/608ee632-9b6d36e6-0286-b47af13db000/log
As one line this will set the log directory to the directory created above.
Make active with:
esxcli system syslog reload
Check with
esxcli system syslog config get
Everything should be good now.
(This is not professional help. I just wrote this blog entry because it annoyed me that this could not be found anywhere. If you use vSphere you need professional help.)