Setting up usernames & passwords should not adversely affect performance, although you could create a situation where the backup simply fails due to a lack of access, especially with Vista - the NAS uses linux and SAMBA to provide interoperability with Windows and there have been compatibility issues - there are work arounds but not being a Vista user, I can't be more specific.
Oplocks - or opportunistic locking should not be an issue for you on or off unless you are trying to access the file from multiple locations simultaneously - it's purpose is to prevent concurrent write access.