Re: How to improve the Squier VM Jaguar Bass HH
Welcome to the forum.
The pickup in your version of the Squier VM Jaguar Bass is, effectively, two Jazz Bass single coils mounted right next to each other, under a single plastic cover. (Think of it as an attempt to be a Musicman Stingray pickup.)
American-made dual Jazz Bass pickups exist but they are expensive and would need to be ordered especially.
Simply putting a regular pair of Fender Jazz Bass single coils under the Squier cover will not work. Conventional bridge and neck position Jazz Bass pickups are of different lengths and have their pole pieces positioned differently to suit the string spacing at their respective mounting positions.
Closed cover Jazz Bass pickups might be the answer.
The pickups installed in Fender Standard series Jazz Bass models used to be of equal (short) length. EMG produced a pickup designed to retrofit directly in Mexican-made Fenders.
Anything else will probably require modification of the pickup cavity.
If the rout measures 3.5 x 1.5 inches, you are in luck. Various EMG and Seymour Duncan Blackouts for Bass "soapbar" bass humbuckers would drop straight in.