Hi,
It's possible to change it by changing the name of the tax rate (via the "manage tax rates" button of the System>Taxes menu), or using a translation override on the name, but in that case, it will affect all the areas (the emails, the order details in the backend, in the frontend, the invoices, the checkout, etc).
If you only want to do the change in the email, it will require modifying the code of the email.
In the preload, you'll find the line:
'NAME' => hikashop_translate($tax->tax_namekey),
That you could change to :
or to:
'NAME' => 'VAT '.($tax->tax_rate*100).'%',