How do I make a field on the vendor signup form optional, or remove it?

Edit the field under Vendor Form Fields.

1. Go to Vendors → Vendor Form Fields.
2. Click Edit on the field.
3. Untick Required to make it optional. To remove it from the form completely, untick Active.
4. Click Save.

The seller signup form and your own Add Vendor form both follow these settings. Step decides which page of the signup form shows the field: 1 for the account page, 2 or higher for the store details page. Use Add Field to ask for something new. Fill in Label (required) and choose a Type.