/vendor is sister partition of /system on Android devices which holds system applications and libraries that do not have source code available on AOSP but added by vendors (OEM's). It also contains SoC firmware images i.e. hardware specific libraries and binaries (OpenGL, ISP...).
Proprietary blobs (HALs) usually live in /vendor as shared libraries (.so files) which are loaded by Android binders when processes call a hardware component. This partition was optional before Treble support and /vendor used to be just a symlink to /system/vendor. However, on Treble devices it contains a separate filesystem, usually same as that of /system i.e. ext4 or f2fs.
If you accidentally flashed recovery image over vendor partition, it won't brick your device unless you have a ROM or factory firmware to flash. However its filesystem is erased which needs to be recreated. Factory firmware flasher will do this for you but you can also manually format it from TWRP Wipe menu or through terminal emulator using mkfs.ext4 or mkfs.f2fs commands.
Further reading: ANDROID DEVICE PARTITIONS and FILESYSTEMS