please suggest me the best way to get the data from parent site to sub site using Lookup.
Thanks in advance!
What you need to do is create a site column at the top level site (IT) which is a lookup to your vendors list, then in your sub site you add the site column into your custom list.
Please refer steps mentioned article.
The best way that I know of is to create the lookup column on the parent site as a site column. Go to this url (Assuming youre working on SP2013):
Once the lookup is created you can choose it on both the parent site and on the subsite.