VAT is a universal tax on goods. A tariff is basically a tax that applies only to imported goods. So a tariff distorts the market, making imports from a region more expensive relative to other regions, or domestic goods.

Note that basically any tax is bad from an economic perspective. However for the government to function revenues must be raised. It is considered better for market efficiency to raise revenues in such a way as to least distort the market. Tariffs are a very distorting instrument, VAT is generally considered less distorting because it affects all parts of the market equally.