Well our dashboard page is nothing but built with widgets . If somehow you can add our widgets to vendor dashboard page , that would be great .
Apart from this in wplms user roles are not strict .for instructor user role : anyone who has c
apability : "edit_posts" can be an instructor in wplms and I guess vendor user role should have this capability , So a vendor is also instructor .
I guess for now it wont be easy to merge both the dashboards .you can add link to vendor dashboard in wplms instructor dashboard .