I had a similar problem, digitizing VHS through a composite video output using EasyCap USB dongle. The bright portions of the output were saturated white, and there was no option to reduce the brightness on the videorecorder.
I split the coaxial cable near the videorecorder, and added a series potentiometer as sketched. 2 kOhm were probably too much, but with a very slight movement, introducing roughly 50 to 100 ohm to the central conductor, the signal was attenuated to ca 50% of original brightness, which was exactly what I needed. The colours were not too much affected; I have read the chrominance is phase-encoded in the signal.
(It required some tweaking. With increasing resistance over 200 Ohm, the signal quality deteriorated, becoming blurred, dim and the colours went weird.)
simulate this circuit – Schematic created using CircuitLab
Everybody would say this is not the correct way to do it. But it was perhaps also the only way, since I found no brightness adjustment in the VCR nor in computer. To conclude, nothing blew up and this utterly nonstandard and amateurish modification has fixed my problem.
EDIT: For comparison, I created screenshots of the recorded videos before and after the modification.