Right, the system baseline (255) starts well below the vendor's max priority (128). The vendor would start at 254 presumably, and overeager vendors wanting to start at 128 would only harm themselves (in terms of future flexibility).

When MS needs to supersede the vendor driver, they can issue priority 253 (i.e. vendor + 1) graciously or 127 violently. They can also retract the 127 driver or overwrite its priority back to 255 when appropriate.

Default to 192 if an explicit priority is not set.