Yes, VAT is a Scrabble word!

Letter values in Scrabble US: V 4, A 1, T 1.

Is it a valid word?

VAT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a tax levied on the difference between a commodity's price before taxes and its cost of production
a large open vessel for holding or storing liquids

Also: value-added tax, ad valorem tax.