I've found that when updating from Library (with only certain options chosen to update), the Vendor Partner Cost is automatically updated even though I have all pricing options unchecked. Along with the phase, labor, and MSRP checkboxes, we need one for Vendor Partner Cost to allow disabling this if desired.

There are times when we're working with new pricing info ahead of time that's not in the Vendor Partner for another month or so or we have manually entered our additional discount on certain product categories that are not factored into the Vendor Partner cost system. These are situations where we would selectively update vendor cost and don't want it automatic.