Hi Thom,
You can change the template, but it only gets used during Provision or Refresh. If you want to change any Oracle parameters in a VDB, just do it via sqlplus/spfile/etc as you would for a non-Delphix database.
When you do change a parameter, it is a good idea to change the template anyway so that you still get the change after a re-provision or refresh, so you don't need to remember to change it again afterwards.
Thanks,
Scott.