The Toggle VIP option provides a level of priority to the migration batch objects.
When a mailbox is marked VIP, it is placed at the top of the migration queue, ensuring that the object is migrated before other, non-VIP mailboxes.
 
The Migration Queue
When a migration batch is built, the order of the mailboxes in the batch are placed randomly due to internal hashing of key data.  Since it is possible for 2 or more people to have the same name, we needed a process to have unique values for each mailbox.
The queue itself is managed simply by file names in a folder on disk.  Priasoft is leveraging the durability and capability of the file system in lieu of using a SQL database.  File names and paths have a length limit in windows and in order to ensure that we never exceed those limits, we hash user info to generate filenames that are always the same length.  This creates a random order to the mailboxes in the batch.
The VIP option marks the queue items in the batch so that they sort to the top while still keeping the consistency of file length.
 
When to use the VIP option
Based on customer feedback, the most common use of the VIP option is as the moniker implies: an important person.  However, the criteria that makes a person important – within the scope of a migration – is usually based on WHEN that user needs to access their mailbox again.
Consider a batch of 1000 mailboxes and the batch is scheduled to start at 8pm on a Friday evening.  If in the batch is a user who needs to do work Saturday morning, it may be important that the user does that work on their new, migrated mailbox.  Marking this user as a VIP will ensure that the mailbox is migrated first in the batch and should complete long before the user would need to access it the next morning.
So, in most cases the use of VIP is to control time and availability.  Mark several mailboxes as VIP when those mailboxes need to be available in the target environment earlier than others.
 
The second valuable use of the VIP option is for mailboxes that have very high item counts.  Consider the same 1000 mailboxes, but in the batch is a mailbox with 250,000 items and a previous dry-run showed that this single mailbox alone will take 11 hours to migrate.
Further, the business has placed a restriction that a the migration can only occur in a 12 hour period from 7pm Wednesday to 7am Thursday – their reasoning being to not have network capitalizing tasks run during normal business hours.
Since the large mailbox will take such a long time to migrate, it is best to mark the mailbox as a VIP so that it will start at the beginning of the migration period.  Without this option enabled, it could happen that the mailbox start 2 hours after the start of the migration and then runs over the allow window of time.