No Van Neumanns are not a GRAND Menace, just a Menace that can become quite dangerous instead of annoying or mildly dangerous. There are things so terrible wandering the depths of SotS space that even in the middle of a knock–down drag–em–out war, enemies could see themselves forming a temporary alliance against something…else. Here is a basic rundown of the different types of "Menaces":
System Confined Encounter Randomly generated upon Map creation, with the number appearing adjusted by a slider during Game setup.
- Does not move from generated location.
- May also be generated by other Encounters.
Mobile Random Encounter
Generated encounters that have a percentage chance of happening per turn, adjusted by a slider during Game Setup.
- Have an increasing chance of occurring every turn that a player is not in a combat.
- Each combat that a player fights decreases this chance.
- Will not (generally) occur if you have other combats that turn.
- Will not occur before turn 25.
- Will occur at a randomly selected colony in your Empire.
- Can be detected by Science Stations one turn before they occur.
Mobile non–Random Encounter
An encounter that spawns from an in–game encounter that already exists, usually a System Confined Encounter.
- May create additional System Confined Encounters (eg: Silicoid Queen).
- Some can be detected by Science Stations one turn before they occur.
Grand Menace Encounter
One per game by default, defined by a slider during Game Setup.
- May spawn Mobile non–Random Encounters (eg: Locust Hive World).
Defeating some of the Unknown Menaces will give the player monetary or research bonuses. When multiple allies defeat one of those menaces, all allies present get the full bonus. It is not split up.
In the case of VN's, they are an automated "mapping" system, that has the capability to "overcome" detriments to its mission (mapping), by removing said impediments of that mission. The greater and longer that impediments remains, the greater their capability to overcome. So you have to balance the idea of stopping them at all costs KNOWING they just keep getting stronger and meaner and approach a "Grander" menace, or let them go about their business (not interfere) and take the annoying "knocks" they give you. Sometimes poking back at the nasty monster in deep dark space, pokes back.

If you can find their homeworld and defeat them there, is the only way I know of to remove that particular menace from bothering you (but good luck doing that!).
You can get a similar situation from the Silicon Queen(s) ( an automated mining system gone a muck ). Meet one and they aren't any big deal, leave them alone (and allow them to spread and spawn with freedom) and you can quickly face a much greater menace then just one or two.

In this case, you want to stop them at all costs, leaving them be means you will have much greater problems later.
As far as I know these are the only GRAND menaces in the game:
Grand Menace
Locust
System Killer
Uuatu Observer
When facing VN's if wish to just keep them to a relatively minor nuisance just remove the "harvesting" units, and leave the Mothership alone. If you destroy the Mothership(s), you increase your "Threat" level to them (impediment) and they come back with something stronger to do the job.

Now, attacking their homeworld (if you find it) immediately puts you at the top of their list of 'impediment that must be removed' list. hehe So be sure to do it as close to one go as you can, or feel great pain.
