Dear Mr. Rajeev
It is correct that if you call the electrician for fixinf the fan or called plumber for fixing the tap you have to deduct the ESI from their payment.
By taking registration with ESI you cover the establishment and any repair and maintenance work carried in your establishment attract the ESI Provision.
If you use the coolie at railway station there is no need to deduct any ESI as he is not working in your establishment. However if you call the coolie in your premises for loading and unloading you have to deduct the ESI from the wages.
Question whether electrician or plumber will accept such deduction, probably no you have to book the expenses in the voucher @106.5 % and have to deposit the contribution with ESI.
If you appoint the vendor to provide you the services as the principal employer you are responsible for the contribution either you vendor will pay the contribution and charge you the same in service bill.
If the vendor does not contribute for ESI because he does not have 20 people in his company even though your establishment is resposible for contribution for the work done in your organization