The history of Navy VX development/evaluation squadrons is complicated and inadequately documented but I've attempted to summarize it. An example of complication is VX-3. In its first (and brief) incarnation, it existed to evaluate helicopters and develop operational procedures for them.
VX-3 was one of four new air development squadrons (VX) that the Navy formed in 1946 to develop and evaluate aircraft tactics and techniques as directed by a command that was a consolidation of fleet units doing development work. (In December 1947 this command was designated the Operational Development Force.) Other squadrons were subsequently added. In 1969, the surviving Air Development Squadrons became Air Test and Evaluation Squadrons.
The first VX squadrons had two-letter tail codes with the first letter being X. In 1957, the first letter of the east-coast-based VX squadrons was changed from X to J.
VX-1 (XA/JA) Anti-Submarine Warfare: VX-1 was originally an Aircraft Experimental and Development Squadron established at NAS Anacostia in Washington D.C. on 13 August 1942 (in 1945, according to Johnathan Clayborn’s comment below, it was an Air Test and Development Squadron). A detachment for aircraft antisubmarine warfare development was established at NAS Quonset Point, Rhode Island on 1 April 1943. This detachment was the basis for the next VX-1, which was commissioned on 15 March 1946 and moved to Boca Chica Field, NAS Key West, Florida. VX-1 relocated to NAS Patuxent River in September 1973 and is the only one of the original four VX squadrons still in existence.
VX-2 (XB/JB) Drone Controller/Guided Missile Development: VX-2 was formed on 15 March 1946 from VJ-20, which existed for only a week having previously been XVJ-25, which was established on 16 June 1945 at NAS Brunswick, Maine to support XVF-200, an Experimental Development Squadron formed at the same time to evaluate and test Kamikaze defenses. In 1956, according to Johnathan Clayborn’s comment below, it was know as an Operational Development Squadron. VX-2 was based at NAS Chincoteague, Virginia until it was disestablished circa 1958.
VX-3 (XC) Helicopter Development: Established at NAS New York on 1 July 1946 and moved to NAS Lakehurst. It apparently didn't take long to sort things out because the first VX-3 was disestablished on 1 April 1948. Its personnel and aircraft were assigned to one of two utility helicopter squadrons HU-1 (UP) and HU-2 (UR) located on the west coast and east coast respectively.
VX-3 (XC/JC) was reincarnated in November 1948 at NAS Atlantic City to accomplish development and evaluation of jet fighter tactics and procedures. It was formed by merging VF-1L and VA-1L of Light Carrier Air Group 1L. VX-3 was relocated to NAS Oceana, Virginia before NAS Atlantic City was decommissioned in July 1958. It was disestablished on 1 March 1960.
VX-4 (XD) Airborne Early Warning Development was established with the personnel and aircraft of VPB-101 on 15 May 1946 at Floyd Bennett Field, New York, flying PB-1Ws (B-17Gs with APS-20 air search radar installed in place of the bomb bay). The squadron made the first hurricane surveillance flight using radar in September 1946. It relocated to NAS Quonset Point, Rhode Island, in September 1946. It subsequently moved to NAS Patuxent River in July 1948 and was reportedly redesignated as Airborne Early Warning Squadron 2 (VW-2) in June 1952.
VX-4 (XF) Air-Launched Guided-Missile Development: Established 15 September 1952 at Point Mugu, California. It was disestablished on 30 September 1994 as part of the consolidation with VX-5 to form VX-9.
VX-5 (XE) was commissioned on 18 June 1951 at NAS Moffett Field, California. The squadron was initially assigned the development and evaluation of aircraft tactics and techniques for delivery of special weapons (nukes) from AD Skyraiders in all-weather conditions. In July 1956 VX-5 moved to the Naval Air Facility, China Lake, CA, since much of their test effort had involved use of the ranges and instrumentation facilities there. Semi-permanent detachments were located at several other bases Over the years, VX-5 maintained detachments at other Navy bases, e.g. at NAS Whidbey Island, WA to monitor EA-6B developments. VX-5 was formally disestablished on 29 April 1994 as part of the consolidation with VX-4 to form VX-9.
VX-6 (XD/JD) Antarctic Program Support: Established at NAS Patuxent River, Maryland on 17 January 1955 and subsequently relocated to NAS Quonset Point from which it deployed to the Antarctic from October to February each year. It was redesignated as VXE-6 in January 1969. Before Quonset Point was closed in 1974, the squadron was relocated to Naval Air Weapons Stations Point Mugu, California, where it was disestablished on 27 March 1999. (At some point, possibly associated with the move to the west coast, the tail code became XD again.)
VX-7 I haven't found anything on a squadron operating as VX-7.
VX-8 (JB) The Oceanographic Airborne Survey Unit was established on 1 July 1965 at NAS Patuxent River. It was redesignated VX-8 on 1 July 1967 and became the Oceanographic Development Squadron, VXN-8, on 1 January 1969. It was disestablished on 1 October 1993.
VX-9 (XE) was formed at China Lake from the consolidation of VX-4 and VX-5 directed by the CNO in June 1993 as a cost reduction measure. It was established on 30 September 1994. It evaluates strike warfare airplanes, weapons, and tactics, to include electronic countermeasures, from an operational standpoint.
HMX-1 was established in on 1 December 1947 at MCAS Quantico, Virginia for the development of amphibious assault via vertical envelopment. In September 1957, it acquired the additional role of presidential transportation via helicopter.
In early 2002, only three "X" squadrons remained: VX-1, VX-9, and HMX-1. Five more were added on 1 May 2002 as a result of redesignations of existing units:
The aircraft test and evaluation squadrons at Patuxent River were redesignated as VX squadrons;
VX-20 (Force) Naval Force Warfare Aircraft Text Squadron (VP, VS, VAW, VX, VR, and VT aircraft)
HX-21 Naval Rotary Wing Aircraft Test Squadron (for some reason, this squadron does not mark their aircraft with a tail code)
VX-23 (SD) Naval Strike Aircraft Test Squadron
Two existing Naval Weapons Test Squadrons were also redesignated:
VX-30 (BH) Naval Weapons Test Squadron at Point Mugu
VX-31 (DD) Naval Weapons Test Squadron at NWAS China Lake