TWRP: vendor partition is not existed
OnePlus 5 didn't support project Treble out of the box, so there was no /vendor partition. However with update to Android P (as explained here), said partition was created to support Treble. If you didn't undergo that OTA update, you must haven't /vendor partition on your device.

You can also create the partition yourself from within TWRP if you are a bit tech savvy. parted or gdisk can be used for the purpose. But playing with partition table could be risky if you aren't sure what you are doing. You can see the partition table of your device in given link.