Each purchase is not necessarily tagged to a single category, you can have multi line purchases with some or all lines assigned to different codes (e.g. I often split general purchases from carriage charges). The only place I can think of where you could extract this would be to start from the purchase items CSV from an account backup, filter that by date in excel and then use a pivot table to aggregate the net and vat by nominal code.
It might be easier to be more diligent with tagging - that way I can get quick summaries in Purchase Management. Column totals in this view would still be a nice feature, mind. At the moment, you only get Gross total, and then you can subtract the Net Total that you get from the tag P&L page