VIP is an extra special bonus you get when you purchase more than 100 firestorm points.
You will receive the following benefits when you have the VIP status active:

• You can access your personal bank at any time with the following command: .vip bank
• .vip capital will teleport you to either Orgrimmar or Stormwind.
• The flights and disconnections will be instantaneous.
• No need to repair (your items won't lose durability).
• Instant mails.
• No resurrection sickness.
• You can gather herbs while mounted.
• +30% more reputation gained.
• +30% more experience gained.
• +30% more artifact points gained. (Legion)
• +30% more honor points gained.

The first time you buy VIP you will get 30 days of it. You are able to renew it every 1st of the Month, NOT, for the day that you bought it. So it is possible for you to extend your VIP for longer than 30 days.

I will give an example of how this work:
Imagine you donate 140 points on June 15th. This is the first donation of the month with at least 100 points, so 30 days of VIP are added to your account (it'll have the VIP status until July 15th). If you donate 140 points again on June 20th, nothing will happen since it's not the first donation with at least 100 points. However, on July 5th, you decide to make a new donation, this time of 700 points. Your VIP then will be extended 30 new days (because it's the first donation of the month with at least 100 points, but you already have an active VIP that will last until July 15th). Therefore, your new VIP will last until August 14th (July has 31 days).