Your application sounds interesting
The VISA driver uses a 8251 compatible chip in the PC. This chip can work fine with sync and async RS232 - USART also. So I think it's no problem to use it to obtain the transmitted data.
Right click on the VISA Write VI, and select Synchronous I/O mode > Synchronous mode
Source article is here: http://digital.ni.com/public.nsf/allkb/ECCAC3C8B9A
By sniffing the data, it is also important to do not disturb the communication between the devices. We should configure somehow a passive mode - don't wire the handshaking signals, and timing signals.
For synchronous data transmition, the DB9 connector doesn't have enough pins. The DB25 connector has the 15th 17th and 24th pins, wich are required in synchronous transmission.
I checked out the picture you sent, but for me it doesn't look like the standard DB25 conector's pinout. I found this website, which describes very well the cablings and pinouts for the different RS232 modes:
The LPT port can be also DB25 form, but I don't think that you can use it also as a Serial COM port. (Maybe some PCs supports it, I don't know) Probably you'll need a PC card with a DB25 COM port.
I hope, I could help you with some informations for your project.
As I mentioned in you previous thread this is not RS-232 There is a link to the manual in the other thread the manual will explain how to connect the eval board to the PC's Parallel port.. NI spy will let you monitor the port traffic.