As suggested before, the VU meters on Acquavox/Nebula are not accurate. You can download the free PSP VintageMeter and set it so that -18dBFS=0VU. You can then place it prior to the Acquavox/Nebula instance for accurate VU metering.
ok, I'll check those meters... they were prepared inside the skin... I thought they were working. You should consider this is a RMS AES17 value, so it could differ from other softwares (based on peak mode, average mode, or standard RMS mode). I never tested it, but a normalized 1Khz sine should retrieve a -3dB value, could you check that?