Vac pump vanes need to form an air-tight seal with the pump housing so they develop the vacuum the pump is supposed to supply.
Old vac pumps did this via an oil film but the downside is the oil contamination of the vac supply. Modern pumps have an aluminium housing which is highly polished. The vane ends are made of graphite (think pencil 'lead'). The graphite provides a soft bearing surface that conforms to the pump housing, thereby sealing the pump but it is fairly brittle material and doesn't like being loaded in ways other than designed (think of your pencil again)
The vac pump is available in clockwise and anti-clockwise models (for different engine rotations), the difference being due to the vanes being angled in the direction of motion.
Turning the prop backward, against the angle and contra to the profile the vane ends have worn to, runs the risk of breaking up the graphite vane ends in the pump, breaking the seal and making the pump u/s.
My view (as an engineer but not on aircraft) is that turning a prop slowly backward as you describe will probably not do any damage. However, the risk is there.
My 2p-worth: props should always be treated as live which ever way you turn them. Backwards is not a no-no, so long as it is carefully done and the risk is understood!
Hope that helps
Edited to add
http://www.avweb.com/news/maint/182905-1.html - which explains still more and confirms the above...