This is a common problem with the VS4. I’ve owned 3 over the years, 2 had the problem and 1 was great. The problem is the large pole pickup, for some reason they loose body and become very bright.
The best fix is to just replace the pickups, I’ve put some Lindy Fralins in and it works fine with the pre-amp. Equally, replace the pickup and replace 2 pots to get the Passive P Bass sound.
it would be interesting to find out why this happens to the pickups, demagnetised?