Make sure that Vista is using your flash memory device for ReadyBoost.
Go to My Computer or File Explorer, clear the address bar at the top and enter this: eventvwr.msc
Press Continue when Vista asks for permission, and the Event Viewer window will be displayed.
On the left panel of the Event Viewer, navigate the tree as follows: Applications and Services Logs > Microsoft > Windows > ReadyBoost > Operational
Double-click the ‘Operational’ section to open it.
On the right panel, hit the ‘Find’ button and search for the word ‘Speed’. Now close the Find window. In the middle panel are Vista’s speed test results (note: ‘Sequential Write’ appears to be a mistype, and is in fact ‘Random Write’)