Does having more cores improve Vista performance and some related Qs?

The short answer is Vista will be subject to whatever software you are using to virtualize it on your Mac. If there is some limitation of your virtualization software, running Vista (or any OS, for that matter) inside it will not solve any problems.

- How many cores does Vista Ultimate 64-bit support?

In consumer versions: One physical processor, core count is unlimited. If you require SMP, then you need to use Server 2003/2008.

- What is the optimized number of cores to run this version of Vista?

There isn't an 'optimized' number - Vista will utilize N number of processor cores.

- Does having more cores leads to improved performance?

It depends on application design: If you are running heaviliy multithreaded apps, then Vista is quite efficient at dividing the workload appropriately. Most consumer apps, however, are not deeply multithreaded. In such a situation, the performance gains from having multiple cores available are limited. But that's not a Vista issue - It's a design issue on the part of the actual application.

- If I run four instances of the same applications and each application
normally takes 3GB, does that mean the total number of RAM I
need will be: 3x4+whatever amount required by vista?

Correct - There is no free lunch, and no operating system on the face of the earth can do what you appear to be asking for.

Will the RAM be distributed evenly automatically?

RAM usage will be automatically distributed according to application needs and system resources.

- Is the assignment of cores automatic?

Yes, core assignment is done by the operating system, and according to application/system usage and needs. This requires no intervention on the part of the user. As a matter of fact, it's very much for the best to not interfere.

Can it be done manually? For example, I want core no.1 for application A, cores no.2-4
for applicaton B, core no.5 for application C, etc.

Yes, it can be done manually - This is not recommended, though, as making this assignment prevents the OS from performing proper load balancing. Making this change is temporary, and will revert back to the default, automatically balanced, settings upon OS restart.

- How do I assign a specific core to a specific application?
(e.g. core1 for instance 1 of application A, core2 for instance 2 of
application A, core 3 for application B, etc.)

How to set Priority: http://www.vistax64.com/tutorials/ [...] esses.html

How to set Affinity: http://www.vistax64.com/tutorials/ [...] inity.html

...as mentioned already, this is not recommended.

Helllo. I read that Vista Ultimate 64-bit supports two physical processors. Does that mean a MacPro with 2 Xeons (8-core) performs better than the same machine with only 1 Xeon (4-core) processor? I plan to run Vista through Bootcamp rather than through virtualization software.

Why would you buy something as overpriced as a Mac just to run Windows? I hope you'll be making actual use of MacOS and not just paying more for a PC than you really need to...