Can I efficiently monitor sound card line in with a vu meter?
Yes, Paul, you are missing something basic: The fact that dB is not an absolute measurement, but a relative one. You need to know what your VU meter is relative to.

Could your VU meter is reading dBm instead of dB? dBm is a measurement relative to 1 milliwatt, with 0 dB being a milliwatt. If not, then the VU meter is relative to something else, specific to the application it was designed for.

As an example, the VU meters on a power-amplifier are usually relative to the maximum continuous power, with 0 dB being equal to 100 watts for a 100 watt amp. So most of the scale is negative dB, with the positive dB being in the clipping region (an aside: it might be an issue different than clipping that sets the maximum continuous power for the amp). With +3 dB as your VU meter's maximum, it seems like it is a relative reading, and not dBm.

In audio, most dB measurements are relative to full-scale, so you are almost always dealing with negative values.

If you can determine what the VU meter is measuring relative to, and also determine what Audacity's dB scale is relative to (possibly 1 volt p-p), then you can determine a constant value to add/subtract from the VU meter's reading to correspond to Audacity's dB reading.