Solid advice on the compression, and I do get where you're coming from, but as someone who does a lot of original content, waveforms actually do matter, especially alignment-wise. I know of so many cases where a mix was not listenable because an instrument or a vocal part was off-time. Not only this can cause phase issues, it can also cause unwanted dissonance. Also, even if the OP doesn't mean equalisation, it's a good habit to use an EQ on vocals anyway.
Back to the OP.
Two compressors at a 2:1 ratio, each doing about 3dB of gain reduction should keep the volume at a consistent level. Then use the make-up gain feature on the compressors. Or, if you're on Vocaloid Editor, use the Gain knob on the top of the mixer channel for Petit to bring it back up to listenable level after the compression. For both compressors, try to go for a medium attack and fast release. In most cases, a compressor's attack and release is based on milliseconds, which means the lower numbers mean faster attack/release. The only exceptions are 1176 clones, but StudioOne's Fat Channel 1176 clone is also based on milliseconds.
Also, on Petit herself, yes, you should do a bit of EQ. Start by putting on a highpass filter. Move it all the way to the left and then slowly bring it back up until you start hearing changes in the timbre. Then move it back down just barely enough so you don't hear the changes again. Then proceed by adding a high shelf roughly around, say... 4-6K? Don't forget to use a de-esser.
WATCH OUT FOR THOSE 300-500Hz AREAS!
WintermintP