There is one thing to consider while talking of reliability or non-reliability of volumes. That is something called sampling.
With no centralised exchanges, the volume data on fx is questionable but it can be taken to be a sample representation. It is like the answers to the questions on any stats based shows or in India Today on "pre-martial sex". Answers might be 35% in favour of doing it. Well do really 35 % of the Indian population think so on the question? They might or might not but sample of 1750 out of the 5000 polled by India Today think so and that is enough for them to proclaim it as the truth. Truth from a sample of 5000 out of 1 billion to make a bold statement.
So, is the volume reliable? To some extent it is, there are people who trade exclusively using VSA on fx and are successful.