At ClueTrust we have had good experiences running from high quality USB sticks, for software that is designed to run off USB or SD cards such as ESXi and SmartOS.
VyOS is not one of those software packages. It appears to try to swap to the usb stick, which results in pain. It certainly writes logs to flash.
What's the pain? Well, on a Dell R620 we eventually noticed complaints about filesystem write issues on the thumb drive popping up on the console. But the main thing we were seeing is spates of watchdog timeouts on various cores. My hypothesis based on some changes we made that made things better but didn't fix them completely is that we had some sketchy stuff going on in bursts that was overwhelming log writing to the usb drive.
Switching to raid1 behind an H310-Mini with default firmware on it fixed that, and the system is behaving much better.
Oh, the high quality thumb drives we've used for booting SmartOS etc. are Sandisk Cruzer Fit in whatever's the cheapest size (4 and 8gb are no longer being made and are more expensive than 16g these days). Available in multi-packs on Amazon.