One solution will be to add a checkbox per vendor and create the relationship via a logic hook of workflow. The problem with that is that you add/remove manually as you add/remove vendors.
The other option is to take a look at the field collection mentioned on this post My changes for SuiteCRM 7.11.10
Based on the description, it should allow you to add multiple related records at the time.
Please take the above as suggestions. I haven’t done anything like that myself.
Thanks,
AlxGr