While plug-ins get the job done they are no match for "hardwired" gear...especially for Vox. However...I tend to use both ...Light compression for tracking...to get smoother consistent signal input. then I add compression as needed in the mix.
Inexpensive plug-ins work ok if used disceetly on Vox...but remember the average listener knows weel what the human voice sounds like...and any artifacts introduced to the Vox track will be noticable. On the other hand with other instruments...the average listener doesn't know if they are listening to a Cheap Stratocaster clone or a $2500 Paul Reed Smith.
As far as plug ins go Waves C1 is excellent, but so are Cubase plugz. The C1 however is very smooth...much like a hard wired compressor.
Out board?? There are many to choose...but I've heard goos things about Behringer Composer...dBx 266...they both run about $250. If you want to add warmth to your mix and Vox...look at an ART TubePAC. It's a single channel tube pre-amp and tube compressor all rolled into one. It's very smooth and fairly transparent...has phantom power so you can use it for a DI box on your favorite Studio condenser mic. Cost about $220 retail...I bought one for $180 at fullcompass.com