Kirk,
Excellent Question.  Configuring the web server was tricky on Vantage 4.2.  We have simplified this procedure for Vantage 4.3.
Here are a couple points for configuring the Vantage ISAPI redirect for Vantage 4.2: 
 - On a 64 bit machine, the 64 bit ISAPI should be used.  It is located in 'vantage/gateway/isapi/isapi_redirect_x64'.  
 - The 64 bit ISAPI redirect must run in a 64 bit only app pool.  To achieve this: 
   a) Create a new app pool
   b) Move the jakarta application to the new app pool
   c) Set the property 'Enable 32-Bit Applications' to false (under Advanced Settings).
Have a great week,
Jake