The VAT paid by a merchant is a plus and minus figure.The one who actually pays it is the consumer/or the end user of the product, and in these transactions there is no consumer and no end user. (end user or consumer means the one who depreciates the value of a product down to zero). So if applied on Forex transactions the net amount of VAT payable by the speculators will always be zero. So no a VAT on those transactions is meaningless as it it will collect a big zero sum.
A direct tax on Forex transactions yes, it is applicable, but you cannot exclude from that very same tax, the transactions made for real buying of foreign goods and services. Eventually it will have another zero sum effect on speculating transactions because the speculator would either sell his money to buyers of real goods and services or to other speculators.The tax you impose will just be part of the value of the forex money.