The base is not easy to find; there's a lot of tours on sale to visit the military wonders of Vis, but little actual information. I discovered the location of the base by looking at Google Maps satellite photos, but the Secret Croatia blog also helped...
On the there are eerie rock "tents" that disguise ventilation shafts and escape stairs to the large bunker under the hill. The escape stairs can also reached from inside the bunker, but I did not dare to climb them on this visit. They look inviting, however :-)
The bunker itself is a large, curving tunnel designed for large mobile missile launchers. Both ends have been covered by wires and painted styrofoam.
At the middle of the tunnel there are side roads and bombproof doors leading to odd, tile-floored rooms. Their purpose is a mystery.
If you are interested in visiting, look west from Podhumlje on Vis. The access is difficult, I decided to park my car and walk kilometres instead of getting stuck on the way back, like another car did. The walk in the falling darkness made my trip even more eerie than it already was.