Virtual memory is used off of every partition, you are going to have a page file on the C drive no matter what, you can't get rid of it. I see no reason at all to create a separate partition for virtual memory.
How does fragmentation have anything to do with page files/virtual memory? Putting in another partition would make the drive fragment faster, because it would have less space to put files in.
But if you put another partition to use as the page file, defragging it wouldn't do any good, because it is used as RAM; hence the data is constantly in and out of the hdd.