What is the difference in running the Vuser as a process and as a thread? Which is more advantageous way of running Vuser?

When Vuser is run as a process the same driver program is loaded into memory for each Vuser. This will take a large amount of memory and will limit the number of Vusers you can run on a single generator
When Vuser is run as a thread, only one instance of driver program is shared by given number of Vusers.  You can run more number of Vusers on a single generator using the multi-threading mode.