I am assuming you are getting an error stating that the print driver for 64bit is not installed in the Vista machine. You can either install a 64bit driver that has the same name as the x86 print driver on the Win7 machine and reattempt the connection, install the 64bit driver that has the same name onto the Vista machine (for other 64bit machines) but this is more difficult.
You can also configure the machine to check on Windows Update for a print driver that has the same name as the 32bit print driver. If gpedit.mcs launches for the Windows version you have.
Computer Configuration
Administrive templates
Printers
Extend point and print connection to search windows update
If there is no matching driver name, you cannot make an RPC connection but you can create an SMB connection. Search for Local Port. There are many posts on this.
Alan Morris Windows Printing Team Search the Microsoft Knowledge Base here: http://support.microsoft.com/search/Default.aspx?adv=1