I just bought a Viscous Vibe and I love it. However I ask myself if I have to put it before or after my distortion pedal.
Both options are possible. Depending the result you would like. It will be exactly the same with wah-wah or phase pedal for example.
The Viscous Vibe before overdrive/distortion will give you a subtle effect as it will be less pronounced than after the distortion. The distortion acts as compressor and as filter. The effect levels out the peak in the frequency response of the Viscous Vibe. In other words, putting it before is equivalent to making it less able to do what its designed to do. The Viscous Vibe will not change the overall tone of the distortion If you want to sound like Hendrix, this is the way you have to connect it.
Putting the Viscous Vibe after overdrive/distortion pedal will make the effect more efficient. The distortions overall tone will be modified with pronounced change in frequencies. This puts much more emphasis on harmonics. There is much volume change than if you put it before distortion. However, it creates far more change in the sound of the overdrive/distortion.
Things will be different for modulation, delay and reverb. Most of the time these effects will take place after overdrive/distortion. Uncommonly, some guitarists like to put modulation effects as Chorus, Flanger (sometimes delay) before overdrive/distortion.
For instance see Soren Andersen video with the Vortex Flanger
This sounds like the first Van Halen album.
If you are relying on your amp for distortion/overdrive tones, you can plug your Vibe (or other pedals) into the amp input, which would put the effect before the distortion, or if you amp has an effects loop, you can put it there to have the effect after the distortion.