Based on my interpretation, the answer to “Doesn't FAR part 8.004 list GSA Contracts before Open Market Sources in order of precedence?” is yes in accordance with FAR 8.002(a)(1), not FAR 8.004.

My interpretation of the question “When a Vendor holds a GSA Contract for an item of supply and they are the only vendor listed that holds such a contract, is there any requirement to seek additional quotes for that item on the open market?” is based on the DFARS.  In accordance with FAR 8.404(d), you would not, but a Class Deviation for DoD was issued in 2014 that replaces that paragraph with additional price determination requirements.  DFARS 208.404 instructs you to see DoD Class Deviation 2014-O0011.  This CD requires you to make an individual determination of fair and reasonable price beyond the determination of fair and reasonable price made by GSA.  Since there is only one vendor in GSA, you would likely need to conduct market research on outside vendors to make this determination.  In addition, if you find based on your market research that the GSA quote is not fair and reasonable, you would likely be able to justify awarding to another source outside GSA.  Before making your determination, be sure to ask the GSA vendor for a discount.