Chris,
I think you have a tall-order on your hands here. What you have here is a situation where reimbursable expense payments were misclassified as 1099 type transactions for a particular vendor. Which means, users accepted the defaults, based on the Vendor setup, instead of editing the 1099 amount box to zero for these payments.
Mariano Gomez, posted an article about using SQL Scripts to update detailed 1099 information; however, his queries update all amounts based on Vendor defaults, so it is not an exact fit for your problem.
dynamicsgpblogster.blogspot.com/.../1099-reports-devil-is-in-details.html
As with most situations, using SQL to update your accounting data is not the preferred method. PSTL doesn't have a tool for this job. You would need someone with the requisite skills to update each document for this vendor, which shouldn't have a 1099 amount.
I would strongly suggest using the native feature in GP to edit the 1099 amounts, and pass on updating the documents. I would also teach users to edit the 1099 amounts that default on transactions, that are not 1099 income, or using a different vendor record for reimbursements.
Here is a really good post on using native GP functionality to fix 1099 information.
www.syvantis.com/.../Post.aspx
In any event, make sure you have a backup before attempting any changes in your production system