The actual VSI reading is taken either from barometric pressure or GNSS, and the source is flagged in a bit in the ADS-B packet. Then it's encoded in one of two forms in a 9-bit value with a single-bit sign.
If the 9-bit value is 0, there is no vertical speed available. Otherwise the vertical speed is encoded as +/-64 * (value - 1), giving a range from 0 to 510 for a range of bitfield values from 1 to 511.
Now this gives you a range in 64ft/min steps up to 32640ft/min. Most civilian high-performance aircraft will be in an absolutely screaming climb at around 10000ft/min and an 8000ft/min descent would be pretty aggressive - autopilots may drop into a 8000ish ft/min descent if there's a loss of cabin pressure, for example but 6000 would be considerably more comfortable. So even with your 9-bit value you've got enough resolution to show a fairly gentle climb (try staying below 1ft/sec climb or sink when flying by hand!) and something well in excess of what you'd normally want to experience.