I am wanting to display items with and with out tax. I have edited the product.page as per https://help.shopify.com/themes/customization/products/show-vat-prices#product
This works fine for products without variants. For products with variants as soon as you click on a variant, the with and without tax disappears. I am not sure where the code is that is displaying the variant price?
It seems you're pretty comfortable editing theme files yourself. But in case you might ever need it, we have developed an app called Multiple Pricing which let's you show VAT included and excluded prices out of the box.
Have a look if you're interested: https://apps.shopify.com/partners/developer-2cf34194a44207d7
Thanks and have a great day!