Hi Kate,
You can use the Taxes feature within Cvent to add either a percentage or a fixed amount (in this case it would be a percentage since VAT is typcially 20%) to agenda items. Then run the Order Details report in Reporting > Reports (New) to see the itemized breakdown of each registrant's order includuing how much they paid for tax. There is a great article in the support community called Adding Taxes that highlights the steps needed to set them up.
I hope this helps!
Thanks,
Ayanna