Do you think prequalification of vendors for supplies and services is usefull technique, if yes then how?

Yes, prequalification of vendors and manufactures is a very useful technique.  The process of prequalifications shortens the process of procurement through obtaining, and evaluation of bids from those vendors only, who have the requisite capability, capacity and experience to produce and supply goods and services matching the requisite standards. Once a vendor has been prequalified, the organization is just required to consider the technical and commercial features of the bid, and need not to repeat the porcess of qualifying the vendor.

Yes. To know the ability of the vendor / service provider if they are capable to meet the requirement.