The phrase VV (or "vee vee") isn't funny by itself per se...it's mostly Howerd's delivery of a new joke phrase..which could in part be comparable to "VE Day"...but as I said, it's mostly the delivery...but you can watch it for yourselves.
As for the the double V from "IV V", that's not what is funny...it's the V-sign that Howerd makes when counting 5.
The V-sign in British culture is the equivalent of the US "middle finger" salute and is the physical embodiment of "eff" you/off.
The insulting version of the gesture (with the palm inwards Reversed victory hand) is often compared to the offensive gesture known as "the finger". The "two-fingered salute", (also "the forks" in Australia) is commonly performed by flicking the V upwards from wrist or elbow. The V sign, when the palm is facing toward the person giving the sign, has long been an insulting gesture in England, and later in the rest of the United Kingdom, Ireland, Australia, India, Pakistan and New Zealand] It is frequently used to signify defiance (especially to authority), contempt, or derision.