It is the vendor's responsibility to send the W-9 upon your request. If you do not receive a W-9 before the Feb. 28 filing deadline for 1099 forms, you can still file your 1099s with a notation that the vendor did not provide a tax identification number. You will not be penalized for the vendor's failure to submit his information in a timely manner.
If you do not have a valid W-9 form showing that the vendor is incorporated or affiliated with a government agency and therefore exempt from 1099 reporting, you must take 28 percent backup withholding out of any future payments due to the vendor and send it to the IRS. You do not have to begin withholding until you receive notification from the IRS that the vendor's tax identification number is not correct.