Can I ask vendors for extra details like GST number when they register?

Yes. Go to Vendors → Vendor Form Fields and click Add Field.

• Label is the question vendors see; Key fills in automatically.
• Type: Text, Number, Email, Select, Radio, Checkbox, Date, File and more. For Select and Radio, list the Options (one per line).
• Step decides which page of the registration form shows the field: 1 is the Account step and 2 is the Store details step. Tax and bank details usually fit on step 2.
• Group (Basic, Store or Bank) and Sort order set the order of fields within a step; vendors don't see the group as a heading.
• Tick Required if vendors must fill it in, and keep Active ticked.

Click Save. The field then appears on your vendor registration form.