I've already enabled comments module.
For my website i've following requirements:
- Customers will leave comments about vendor service on Pages
- Vendor will be provided a login who can reply comments
- Only vendors will be able to reply comments
Also I want to restrict some words used in comment author names like "customer".
All I want to whether any module exists with this functionality or I need to do coding myself.
This is almost certainly a custom code job. Drupals default comment system allows anyone to with the correct permissions to comment on nodes.
comment_reply() will give you a starting block to achieve what you want.