> The output I want is a table with
> 
> item_id | title_txt | category  // grouped by category
> 
> Should I use Keys instead?

Yes: you should be using Muenchian grouping. The construct

//vform[not(Category=preceding::vform/Category)]

is very inefficient.

If the vform elements are at the same level in the tree, you could get a
good boost by using preceding-sibling instead of preceding.

Michael Kay