Output troubles...

Rather than fight driver problems, I'd connect your monitors to the VS-100 main outs using an adapter cable: 1/4" phone to RCA phono.  This will also work if you use the VS-100 in standalone mode (which outputs only to the main outs.)

Normally an unbalanced connection like this to powered monitors isn't recommended, but the MA-15D monitors have a 2-wire power cord, so you're not likely to have ground loop problems with this connection.

Murray

The other outputs could be used to drive custom monitor mixes (if you had a headphone distribution system), click monitors, or to act as sends to outboard effects units for example.  These outputs are probably not needed by most home users, but are real handy when you need them.

Deciding on the best monitors for you is a big topic.  I probably couldn't do it justice, but do some searching on the other threads.  Lots of advice and controversy. 

Ground loops are much more likely when you connect two or more devices with their chassis grounded to the electrical ground (third prong on the plug.)  Whether you actually have problems depends on a lot of things. 

Before I answered you earlier, I looked up the specs on the MA-15D monitors on RolandUs.com.  The image of the rear of the monitor shows a two-wire AC connector.  Unless your model is different, I don't understand how you can use a 3-wire power cord with it.