Correct all prices must be inclusive VAT.
The app will most likely use the price you set in the product. Prices modified by 3rd party apps will most likely not be used.
So in Shopify > Product > click on your product > the product here or the variant product price is what is used.
If in doubt get in touch with the data feed app support team you use. Not every app is the same and there are hundreds of data feed management tools. All with there unique features.