OK, I used to run a vB site so I did a little research. It is a vB4 problem, the showpost.php script was crippled allegedly for SEO reasons. Fine. Presently all calls to showpost.php on Phun redirect to showthread.php, and this is the main thing that needs to stop -- that and a bit of repair work to the showpost.php script itself, and the page template that builds the link attached to the little post number on the far right side of each posts blue title/date bar.
I tried various uses of &perpage=1 appended to the showthread.php call and none of them worked, BTW!
Here is the answer, it is a bit of work but I think the benefit could be worth it!
The page showpost.php is all updated to vB4 coding and included in the vB4
download package. It has a couple of areas you need to comment out that 1)
does a redirect (around line 120) and 2) prints as an xml if it does print out
(at the end). So, both those areas need to be commented out. Then, just add
the $templater code at the end to spit out a template, add the template name
to the $globaltemplates array at the beginning of the page, and create the
template and viola! You have a showpost.php page. You would then want
to edit the postbit templates to link to the showpost.php page instead of
the showthread.php page when you click on the post number.