Bring up the app in the Play Store, tap the three dots in the upper RH corner (overflow menu), then uncheck the Auto-update option.
Sideload the OTA image via the instructions here...
I had the same question, and found the answer in the Pure Nexus ROM FAQs...
3.4 Q: What is the vendor.img/vendor.zip? (5X and 6P only)
3.4 A: The vendor partition is new to Nexus phones with the 5X and 6P. Previous devices had the vendor files...
Full RUU Build 4.03.605.2 For The HTC Rezound
Both OEM unlocking and USB debugging need to be enabled in Settings/Developer options before the bootloader can be unlocked.