Can I use the VS100 power supply to power a guitar effects pedal simultaneously? I want to keep the number of power supplies down for live use and I have an adapter to split the power cable. Just want to check it's safe to do. It's a Boss Re20 pedal which I believe is the same spec.

I don't think you should do this - you don't want to starve the VS100 from power in any way. I'll daisy chain guitar pedals, but I wouldn't daisy chain an interface with anything. (just my opinion)
 
If you do try this - make sure both devices require the same specs (volts, polarity, etc...), and that the power supply is rated high enough to deliver the correct power to both at the same time. (ie: The VS-100 requires 1 amp. If the RE-20 also requires 1 amp, your power supply needs to be rated for at least 2 amps)
 

shambolic
It's a Boss Re20 pedal which I believe is the same spec.

 
There cannot be any guessing
An error here can cost you a power supply, or worse - a device...