Yep, use the tools that are best suited to the product you are working. The decentralized nature of forex prevents the computation of a true level of volume. Even price comparison's between brokers vary (and with some, they can vary quite substantially - I've seen variations of over 10 pips in some cases). But those are usually associated with less reliable retail shops.
We probably shouldn't be too quick to dismiss Sledge's strategy of using a volume indicator on forex if it's working for him. You use what works, and if you can find an edge using a volume indicator, then good for you! I consider many (if not most) of the indicators out there lagging in nature and therefore they don't fit in well with my own personal strategy. But for others, they form a fundamental basis and have led to success.
In forex, as in any other trading activity, you won't be successful until you can find your own "edge." If mastering
VSA accomplishes that for some people, so be it.
VSA can be correlated to enhanced volume in forex, although it can also give false signals. If someone can differentiate between the two, then that may formulate the basis of an edge. And that's all it takes to be profitable.