Yes, I've actually seen that before and for some reason it only involved a Vista machine trying to access a printer shared from a Windows 7 machine. Here's what we had to do to get around it, I don't know why for sure, but it seems to do the trick, try the following and then see if you can successfully add the port, and keep us posted.
On the Windows7 machine hosting the shared printer
Go here C:\Windows\System32\spool
Right click on the PRINTERS folder and choose properties
Go to the sharing tab and then click the advanced sharing button
Check the box that says "Share this folder"
Then click the permissions button
Make sure "Everyone" is selected in the top frame and
In the bottom frame check the box for full control under the allow column
Click OK
Click OK again
Now under the Security Tab
Click the Edit button under the top frame
In the next dialog box that appears select the add button
In the next dialog box that appears select advanced button
In the next dialog box the appears select the find now button
In the bottom frame choose "NETWORK" and click OK
then click OK again on the box that you go back to
And then on the next box you go back to click to select "NETWORK" which you just added in the top frame
And check the box for full control under the allow column in the bottom frame
You will probably get a security warning, just click yes
And then you'll go back to the properties box, just click close.
You may have to reboot, not sure. Won't hurt
Now see if you can add the local printer port from the vista machine as you tried before