For installing Windows Vista, the partition must have atleast 10 GB of free space, a typical install will take 7-7.5 GB and if you install software like Visual Studio 2008, it will eatup this space very fast. I recommend you to install Vista on a partition with 15GB or more free space for working practically on it.
To convert your logical partition into primary, you'll have to delete and recreate the partition,as far as i know. Also, logical drives are not created directly. They are created on a Secondary Partition, so you'll have to first delete all the logical drives, and then remove the Secondary partition. After that you'll be able to create a primary one.
One question, why do you think you need a primary partition? Its not required to install Vista. Your partitions must not be having enough free space which is causing them to be incompatible with Vista install.
