Working fine testimonials on ESXi 5.1 as Guest OS Win 8 (64 bit) w/1 CPU, clean 8.1:
First test "keep nothing/clean install": ran all updates, then ran the same "Windows 10 Media Creation Tool" (MediaCreationToolx64) via vSphere Client console, just "Upgrade this PC now" (without building media), worked fine, OS license activated. This was 8.1 with Office 2013, 2GB RAM, 40GB disk with 17 GB free, and I selected to "keep nothing" (effectively a clean install - lost office like an idiot). There was 6 GB free disk space when done, and 800MB RAM in use on win 10 startup. Then, I just installed VMWare tools no problem. Disk cleanup "Cleanup System Files" offered 13.3GB "Prev windows" and 6.5GB "windows install" disk space recovery, and it delivered resulting in 25.5GB free
Move VM: I shutdown the win 10 OS, copied to FreeNAS iSCSI, added to inventory on a different ESXi box, and booted. Worked fine, still OS license activated. Both white box ESXi have same ASUS M5A99FX PRO R2.0 motherborad and 32GB RAM, but src was AMD Phenom II X6 1045T, while dest was AMD FX-8350 Eight Core.
Second test "keep users, settings and apps": a month old 8.1 that's been "off", did not waste time running windows updates but went straight to the same "Windows 10 Media Creation Tool" (MediaCreationToolx64) via vSphere Client console, just "Upgrade this PC now" (without building media), worked fine. This was just clean 8.1, 2GB RAM, 40GB disk with 27 GB free (no Office), and I did it keeping all settings and applications. Same 800MB RAM in use on win 10 boot, but 17GB diskspace free (no Office). Under ESXi, with "keep apps", I did not need to re-install VMWare Tools as far as I could tell. Disk cleanup "Cleanup System Files" offered 2GB "Prev windows" and 7GB "windows install" disk space recovery, and it delivered resulting in consistent 25.7GB free. Seems Win 10 base uses 15GB (maybe 1GB more than Win 8.1).