The thing you are talking about are our in-game tags. This practically has got nothing to do with VTCs, however many people do use it to represent their position in the VTC, or just the VTC in general.
To get your own tag, you can open your MP settings by clicking on the cogwheel on the tab menu. This will open the settings, and you can see the configuration for your own in-game tag there. You can give it any colour you want as well!
Please, do remember certain words are not allowed. For example, swear words or words like "Staff", "Police", "Admin" or any name of a rank in the team are not allowed. If you are unsure whether your tag is allowed or not, be sure to contact a member of staff to ask.