[Bruno]

No?

OK.

The base "Vanish at the start of combat and reappear behind someone" thing is -5. The Backstabber has bought off that -5, and only that -5, and can't buy off any of the other penalties.

In an ideal situation - shadowy tunnels, ambushing the other guy, no encumberance, etc etc, the back stabber is rolling at a total of +5, it breaks down as:

-5 Vanishing
+5 Backstabber
+5 Ambushing
+0 SHadowy tunnels.

Worst case scenario is being surprised somehow in a featureless plain with bright lights:

-5 Vanishing
+5 Backstabber
-5 Surprised!
-5 Not brush or shadowy tunnels

for a total of -10 (not counting encumberance).

The backstabber can be said to have +5, but that JUST offsets the -5 for Vanishing.