Vaccuum 0.0 bar readings on the trip computer

[SuffolkD at aol.com]

Quick question:
I always watch the (turbo) boost gauge while driving.
While driving 1000 miles last week the (turbo) bar gauge showed 0.0 and 0.1
for the first time, while in 4th gear going down a step grade (7%?) for a
short period.
Usually, lift throttle on the highway from 80 MPH, gauge will go to 0.2 / 0.3
bar.

Did I see signs of troubles ahead / maintenance needed or is 0.0 within
specifications for the range displayed while the engine is running?

Side bar info:  The alt belt (loose) had slipped from time to time during the
journey (8v-14v and the abs off light activated and the battery symbol on the
dash lit up) BUT NOT during the event described above.  MPG ~23
TIA
-Scott in Boston