[Motorola One Vision Guides, News, & Discussion] obs3rv:
Answering my own question, the cid of a Motorola One Vision bought off Amazon Italy turned out to be 0x0032, indicating that its bootloader may be unlockable:

(bootloader) cid: 0x0032

You can boot into the bootloader from a powered-off, cable disconnected state by:

holding down the volume-down key

plugging in the USB cable whose other end is already connected to a computer with the Motorola Device Manager drivers installed

Then fastboot works.

(Why did I buy it from Amazon Italy rather than Amazon UK, taking advantage of the Prime Day deal? Despite the fact that Amazon UK showed the Prime Day deal was meant to run for 48 hours, at midnight, Amazon UK pulled its own merchant listing, leaving only third-party sellers, and so the Prime Day deal became unavailable. Amazon Italy had a lightning deal the following day on the phone for €249.99, which was a better price than I could find anywhere else. So I bought it, having missed out on the Prime Day deal the previous day. Thanks, Amazon. Thamazon.)

Almost full fastboot getvar all output for those who are curious (with serial number, IMEI and battery serial number redacted):

(bootloader) version: 0.5
(bootloader) version-bootloader: MBM-2.2-kane_retail-523afc7-190605
(bootloader) product: kane
(bootloader) board: kane
(bootloader) secure: yes
(bootloader) hwrev: PVT
(bootloader) radio: 2
(bootloader) storage-type: ufs
(bootloader) ufs: 128GB SAMSUNG KLUDG4U1EA-B0C1 FV=0800
(bootloader) ram: 4GB SAMSUNG LP4x DIE=16Gb M5=01 M6=06 M7=01 M8=12
(bootloader) cpu: Exynos9609
(bootloader) serialno: [redacted]
(bootloader) cid: 0x0032
(bootloader) channelid: 0x00
(bootloader) uid: 000003D60F1C577A0000000000000000
(bootloader) securestate: locked
(bootloader) iswarrantyvoid: no
(bootloader) max-download-size: 535822336
(bootloader) reason: Reboot mode set to fastboot
(bootloader) imei: [redacted]
(bootloader) meid:
(bootloader) date: 05-20-2019
(bootloader) sku: XT1970-3
(bootloader) carrier_sku: XT1970-3
(bootloader) battid: [redacted]
(bootloader) iccid:
(bootloader) cust_md5:
(bootloader) max-sparse-size: 268435456
(bootloader) current-time: "Tue Jul 23 6:59: 8 UTC 2019"
(bootloader) ro.build.fingerprint[0]: motorola/kane_reteu/kane_sprout:9/
(bootloader) ro.build.fingerprint[1]: PSA29.137-17/6875e:user/release-ke
(bootloader) ro.build.fingerprint[2]: ys
(bootloader) poweroffalarm: 0
(bootloader) ro.build.version.full[0]: Blur_Version.29.11.18.kane_reteu.
(bootloader) ro.build.version.full[1]: retail.en.US
(bootloader) ro.build.version.samsung: 20190510
(bootloader) version-baseband: S337AP_KANE_SGCS_QB2190425
(bootloader) kernel.version[0]: Linux version 4.14.56-g1007313 (hudsoncm
(bootloader) kernel.version[1]: @ilclbld66) (Android (4691093 based on r
(bootloader) kernel.version[2]: 316199) clang version 6.0.2 (https://and
(bootloader) kernel.version[3]: roid.googlesource.com/toolchain/clang 18
(bootloader) kernel.version[4]: 3abd29fc496f55536e7d904e0abae47888fc7f)
(bootloader) kernel.version[5]: (https://android.googlesource.com/toolch
(bootloader) kernel.version[6]: ain/llvm 34361f192e41ed6e4e8f9aca80a4ea7
(bootloader) kernel.version[7]: e9856f327) (based on LLVM 6.0.2svn)) #1
(bootloader) kernel.version[8]: SMP PREEMPT Wed Jun 5 08:01:45 CDT 2019
(bootloader) bootloader.git: MBM-2.2-kane_retail-523afc7-190605
(bootloader) frp-state: protected (210)
(bootloader) ro.carrier: reteu
(bootloader) current-slot: b
(bootloader) running-boot-lun: 0
(bootloader) running-slot: _b
(bootloader) slot-suffixes: _a,_b
(bootloader) slot-count: 2
(bootloader) slot-successful:_a: Yes
(bootloader) slot-successful:_b: Yes
(bootloader) slot-bootable:_a: Yes
(bootloader) slot-bootable:_b: Yes
(bootloader) slot-retry-count:_a: 7
(bootloader) slot-retry-count:_b: 6
all: listed above
finished. total time: 0.611s

FYI Amazon Spain seem to have had this handset at a decent price since Prime Day, and the price hasn't gone back up yet -- the price is down €40 from its usual €299.99 price to €259.99 since last week. Not quite as good as the €249.99 Lightning Deal on Amazon Italy from a week ago, and nowhere near as good as the £199.99 Amazon UK Prime Day deal, but still a good deal. (On Amazon Italy it's €269.99 starting today, but on the other Amazon Eurozone websites it's still €299.99, and in the UK the Amazon usual offer of £269.99 is back.)