The photo doesn’t show it, but this is common knowledge one you understand what a cell’s maximum and minimum voltage. Absolute max voltage of a lithium cell is 4.2 volts unless otherwise stated by the manufacture. I don’t know how many cells in series your battery is. You said you “went down hill with full battery,” this says 4.2V per cell to me.
Well, that explains what is in the photo. Who made it? With pre-made, this is a different story and does not use a VESC controller unless stated by seller. I own a premade regarding experience. If the ESC doesn’t have an usb connection port there’s not much I know you can do unless you’re good at diagnostics stuff with an oscilloscope, but that’s for experts. I will have others chime in so it is not help from one person. They might know something that I don’t.
Just for reference, VESC uses a USB mini-b cable which I think you have if you have a stockpile of old cables.