vTiger 5.4 introduced an option to select the default value for most of the fields. To do so, do:

• Mouse over the “gear” icon on the right upper corner
• Click “CRM Settings”
• Under “Studio” section, hit “Module Manager”
• Click on the wrench(far right side) next to the module you want to add custom field.
• Click “Layout Editor”
• Find the field you want to set default value for, click on “Note with a Pen” next to the field.
• Select the Default value, hit save.

Now every time you create a record, it will default to the value selected.

If you are on vTiger 5.3 or lower you will not be able to set default values using this method.