Lawmakers cannot be law breakers. This is even more crucial when it comes to being chairman of the Home Affairs Select Committee which influences the drafting of criminal laws.
Jeremy Corbyn said on Sunday that he didn’t believe that Keith Vaz had broken the law so it was a “private matter”.
The CPS prosecutors guidance for the Misuse of Drugs Act 1971 is relevant here
Evidence of intent to supply
An intention to supply may be proved by direct evidence in the form of admissions or witness testimony, for example, surveillance evidence …
In addition to the supply of a controlled drug, section 4(3)(a)-(c) of the Act creates offences of offering to supply, being concerned in the supply and being concerned in the making of an offer to supply.
An offence of offering to supply can be prosecuted simply by proving the existence of an offer. The prosecution does not have to prove either that the defendant intended to produce the drugs or that the drugs were in his possession. The offer may be by words or conduct (R v Showers  Crim. LR 400).
Vaz, in offering to buy cocaine for the rent boy, has broken laws which he helped to draft. Even if you take the view that Guido does that this is a bad law, one thing is certain, “Lawmakers cannot be law breakers”…
UPDATE: A previous version of this article quoted from the Psychoactive Substances Act 2016 which came into force on May 26 this year, this also makes it an offence to produce, supply or offer to supply psycho-active substances – this counter-intuitively does not include cocaine.