It is most often seen in the hands of the Riot Control Corps, in the form of a reproduction-variant named the VASH VCRB.
The first Vector prototype was created by Transformational Defense Industries (TDI), later renamed to KRISS USA. This prototype had a fire rate of 1500RPM, rather than the 1200RPM that all later variants hold. It utilized a special off-axis travel mechanism where rearward motion of the bolt is dumped into an inertia block that slid downwards instead of backwards. This was dubbed the "Super V" action. It served to counteract nearly all muzzle climb and sent very little recoil impulse into the shooter's shoulder. The action of the Vector is said to feel much more like a "down-up" balanced motion rather than the traditional "back and up" unbalanced recoil pattern most other guns had.
KRISS USA marketed several variants of the Vector in the United States of America. These fell into two generational categories.
The Gen I category was chambered in .45ACP and was marketed only to military and law enforcement departments for the most part. It had a 140mm (5.5") barrel, a cutout in the upper receiver just above the barrel for inserting a small flashlight, and was capable of select-fire between semi-auto, 2-round burst and full-auto. A semi-auto-only version of it was designated the Vector SBR ("short-barreled rifle"), and was sold to civilians. Due to some states' laws at the time of the states' union, SBRs were banned in certain states. This prompted another variant to be made by KRISS, called the Vector CRB ("carbine"). The CRB had a permanently extended barrel of 406mm (16") in total barrel length. This barrel was encased in a cylindrical shroud that very much resembled a suppressor, when in fact it was not. Yet another variant was made under the name Vector SDP ("special duty pistol" or "security detail pistol"). The SDP featured the absence of a stock, replaced by a blunt end cap.
The Gen II Vectors held onto all the same variants as the previous generation, however it had changes to the upper and lower recievers and fire control group. Namely the hole for the flashlight was gone, and the stock was made into a collapsible M4 style, while still retaining its folding capability. The trigger was also updated to a pivoting style rather than a sliding style, and the selector throw was lessened from 90 degrees to 45 degrees. The most obvious change (only on the CRB) was the switch from a cylindrical barrel shroud to a boxy rectangular one.
At some point in the 21st century, the KRISS USA company disappeared along with many others along with the corporate overthrow of the United States government. The specific design details of the Vector series were lost, though they happened to be documented well enough on the global web through video files, especially disassembly videos.
23rd Century Reproduction-Variant
Around the year of 2186, Cyberian R&D director Riku Tavash managed to successfully bring back the once-lost Super V mechanism and subsequently the Vector itself. He named it the VASH VCRB, which is the shortened form of "Vash Firearms Vector Carbine". This "reproduction-variant" as Tavash called it, took the Vector Gen II design and built upon it a bit more. It features the same large shroud-like structure around the barrel, however this variant was actually integrally suppressed using a ported barrel and the same shroud but with inner baffles. It also came stock with an accessory rail system that floated around the first several inches of the suppressor. The VASH VCRB quickly began to replace the existing SMGs that the Riot Control Corps had, and to this day is most often seen on RC personnel. Each military-spec Vector Carbine has its inertia block tuned in weight by CMF armorers to give the gun the absolute best balance in recoil and minimization of muzzle climb.
Super V System
The original creator of the self-repeating, recoil-balancing action was Renaud Kerbrat, a French man of former affiliation with the FN Herstal. It was named the "Super V system".
This unusual off-axis reciprocating mechanism utilized a very short bolt travel, where much of the energy of firing is quickly transferred into a weight underneath the bolt and behind the mag well. This weight, called the inertia block, traveled down the lower receiver and impacts the floor of it to give the shooter a feeling of downward pull as opposed to a push back into the shoulder. This downward pull was then quickly balanced by the rising of the inertia block, whilst the bolt strips a round from the magazine and inserts it into the chamber.
A properly calibrated inertia block would cause a firearm using the Super V system to experience zero muzzle climb, in theory. This would lead to a vast increase in precision with less practice being necessary on the shooter's part. Coupled with the short bolt travel, it would also allow for a great increase in fire rate.
However there is a reason why the Super V mechanism is not used on larger, intermediate and full power cartridges. This is because the size and weight of the inertia block would need to be unwieldy and impractical. Balancing muzzle climb is still possible however, as exampled in the AK-107, which uses a similar balanced weight-based system but without using off-axis travel.
The VASH VCRB, commonly used by the Cyberian Riot Control Corps, comes pre-calibrated to the best of the company's abilities. This means that the gun is fired a few times and the weight of the inertia block adjusted to bring the muzzle climb closest to zero. The inertia block is calibrated differently for each kind of cartridge it is officially sold for.
Usage of the VCRB
The Vector in its current state is used majorly by the Cyberian Riot Control Corps, by "close-rangers" as they are called in the corps. These men took interest in the VASH VCRB because of its two-round hyperburst capability, comparable to the AN-94, coupled with the unrivaled muzzle control and counterbalanced recoil. The fact that it was integrally suppressed also interested RC as that meant it wouldn't cause pain at the ears for any civilians standing near. It is a common sight to see RC personnel carrying the VCRB, especially when they patrol inside of buildings. They are known to be deadly accurate with the weapon.
Riot Control is known to use 30+ round .45ACP magazines in their VCRBs. They also tune their suppressors to set the sound pressure level of their muzzle reports to about 130 dBSPL, just below that of the lower percentiles of the threshold of pain, but more than loud enough to announce their presence. Furthermore, they calibrate the weight of the counter-recoil system to best effectively utilize the two-round hyperburst function for the smallest groupings.