We have an issue in Enterprise Portal when creating a purchase requisition the vendor drop down hangs/does not return any records.
Are you at the right company in EP?
Neil, you need to turn on your logging at the IIS level and figure out what is happening. You can turn on debugging as in the Microsoft link that the Google term "Troubleshooting Enterprise Portal Performance" will produce. Then, check the event log.
Or you can check the Enterprise Portal post that I wrote: Part 1, basically repeats Microsoft instructions:
Part 2 is where the good, undocumented stuff starts, but I only had time to write it for Dynamics AX 2012r2. I didn't have time to write a 1500 word post for 2009 though the options are there also. That should help get you started sir.