The
VB is the Vertical Bearing, the decent angle in degrees from your current position to the limit shown just above it on the right hand side. The rate in feet per minute is simply the vertical speed required at your current ground speed to achieve that (straight line) bearing - so, no, it doesn't include decelerations. The FPA is the Flight Path Angle - the current angle of decent in degrees.
If the FPA is the same at the
VB then you will make the next altitude target (note that the target ignores provisos such as "below" etc. so if you have a waypoint ahead with "F200B" in the legs page, and the VNAV has calculated a path that crosses at, say F150, then following the
VB on the descent page will have you crossing the point at F200!)
I use it in two ways:
1. If I want a constant angle descent to the next waypoint alt. target, I simply call up the Descent page and set the suggested VS in the MCP.
2. The aircraft descends at flight idle at a FPA of 3.4 degrees. Knowing this, if I am on descent, and above the VNAV suggest path (common with update 10.6, as the default is a constant angle descent between waypoints) then I monitor the
VB, and reduce to idle (i.e. LVL CHG) when the
VB hits 3.4
If the
VB is above 3.4, then I know I need speed brakes.