The problem with speaker-level VU meters is that they will deflect very little at low volumes, peg at higher volumes, or require constant trimming.
IME the better way to run added meters is using an un-mastered output such as TAPE-OUT so that they can be set to actual levels, I have used TASCAM/TEAC meter bridges to do so, I'm sure there are other options as well. If you don't have an un-mastered output, it's really not a VU meter, and you likely should have a different scale not dB.