While we don't offer a setup guide with step-by-step instructions, here is some general information you may find useful.
Whether you need to get a W-8 from the foreign vendor will depend on whether the vendor is an independent contractor providing services and has U.S. source income. U.S. source income to non-resident alien independent contractors providing services is taxable; generally you must withhold 30 percent of payments. However, if you are simply working with a foreign incorporated vendor that is located in another country and doing their work in that country, there is no withholding or reporting (1042S) obligation on your payments to the vendor.
The IRS says "A non-resident alien is subject to tax only on U.S. source income. Every U.S. and foreign withholding agent must file Form 1042-S to report amounts paid to foreign persons from sources in the United States. This includes interest, dividends, rents, salaries, wages, payments to independent contractors, or other gains, profits or income unless specifically exempted under the Internal Revenue Code or a tax treaty. Residents of certain foreign countries having tax treaties with the United States may be entitled to reduced rates of, or exemptions from, tax under the applicable tax treaty."
Furthermore, "If payments for independent contractor services made to a nonresident alien individual[s] are exempt from withholding under a tax treaty or for other reasons, the payor must obtain a signed Form 8233, Exemption From Withholding on Compensation for Independent (and Certain Dependent) Personal Services of a Nonresident Alien Individual, from the recipient before the payment is made."
See IRS Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities (http://www.irs.gov/pub/irs-pdf/p515.pdf). Pages 23-24 define source of income and provide a helpful table. When paying for services, source is determined by where the services are performed. Services performed outside the U.S. are not taxable (or reportable) to the IRS, though there may be a requirement to report payment to the foreign government.
When withholding is required, you want the vendor to provide the appropriate form W-8 (comparable to a W-9), and you report payments on form 1042S. See AP&P2P Network's Forms and Publications of Interest to Filers of 1099-MISC under 1099 Information--1099 Basics (https://www.theaccountspayablenetwork.com/compliance/1099-1042-s-w-9-w-8/1099-reporting/forms-and-publications-of-interest-to-filers-of-1099-misc/). You can learn more about Form 1042 on the IRS website (https://www.irs.gov/).
We recommend you call the IRS international reporting line at 215-516-2000 and provide the exact details of your relationship with the vendor. Be sure to record the information agent's name and ID number, as the IRS upholds the judgments made by their officers.
Note that if you are purchasing tangible goods from the foreign vendor, while there are not federal withholding and reporting obligations, there may be sales & use tax obligations to your state. Also ask if the price includes VAT or not--generally it should not, but if it does, you can reclaim it. More on VAT here: https://www.theaccountspayablenetwork.com/compliance/value-added-tax-vat/.
Regarding set up of the foreign vendor, you want to be sure the mailing address conforms to standards so that correspondence does not get lost in the mail. International address guidelines for most countries are available on this site: http://www.upu.int/en.html.
Finally, your software provider may have step-by-step guidelines for setting up a new foreign vendor, or may be able to refer you to another client or a user group.