If I may, assume these 2 computers are on a network. If so you can try the following.
1. On Vista Bis32 go to Control Panel > Select Classic View on the left side of the screen > System icon > Advanced System Settings> Remote Tab and select the "allow connection". Pick which one suits your needs. Then click Select Users button and add your name in the list so it know to allow you.
2. On the Vista 64 bit start the RDP session and then click the options button and adjust that as you like. I usually click on Advanced Tab then under the Authentication options I usualy select "always connect even if authentication fails".
I need to read up more on RDP that comes with Vista to better answer RDP questions