If the mystery box saber has a Li-Ion battery setup and recharge port installed (which might be pretty rare, as yours is the only one I've heard of...) then the charger you need is this one:
https://www.ultrasabers.com/product-p/z-electronics-1.2acharger.htm It's a Li-Ion charger with a 2.1mm plug that fits the recharge ports that US installs.
It's the 1.2a version, which will allow you to keep the saber on while plugged in
and charging, providing the worlds most awesome night-light. A stand or mount is recommended for this, but a saber with sound, but without a mute function, is not. Here's a couple of the one I made for one of my Diamond sabers (note the recharge cable in the third one). This was basically a proof-of-concept, but it turned out fairly well. There's a couple of things I'd fix but it's good enough until I get around to doing another one
.
The XTAR VC2 is for charging the batteries outside the saber.
No Ultrasabers (at least at this time, AFAIK) use USB as a recharge source (I think mainly because of voltage issues with USB voltage being generally 5v and most US Li-Ion battery setups being 7.4v) so any USB connections you have on a saber soundboard are data only.