Split 3.5mm audio signal
Using a splitter in this way will put the VU meter and the input to your speakers in parallel. As long as the input impedance of your VU meter is high, this won't pose a problem. A traditional (analog) VU meter, is not a low impedance load, so you would want to buffer and drive the meter with appropriate circuitry. A modern commercial VU module, on the other hand, is likely to buffer the input appropriately, in which case you should be able to plug and play.