Deciding which form to display

Can you decid programatically which form should be displayed when adding an entity to a subgrid? I have a Customer Enquiries entity, and in that entity's form, I have a subgrid which looks up to the Vendor entity. The current Vendor form has certain fields on it which are not always required, in particular when 'Add Vendor' is clicked from the Customer Enquiries form.

Is what I want to do possible? If so, would it be done usins javascript or a plug in??

Thanks.