I'm not clear on what exactly is going on - when yous say:
I've checked the led itself and the voltage is getting to it, however the control circuit isn't working by the looks of it.
If the LED die - or LED string/array - is getting voltage, and it is sufficient for the LED(s), then they have failed, rather than the driver.
My query is this:
The datasheet for this VAS1210 states either pwm control or voltage up to 1.2v, however when I've measured the adj pin, it appears it's being applied with ~5v when the led is (supposed to be) fully on and it steps down slowly from there when the LED is dimmed via control. I'm fairly certain the datasheet says the adj pin can handle up to 6v, but is 5v too high or could I actually be reading a PWM signal as 5v?
Hope that makes sense, sorry if it doesn't!
If you're measuring with a multimeter, then it is likely you are measuring PWM - multimeters react/update slowly and can average PWM to DC. Most meters with a frequency counter with duty % which is handy for spotting this. A few photos of the PCB - and the LED's - so we can see how its put together, or how many LEDs are in a string, what type etc.. woudl help a lot.
If there is say, two white LEDs in series, and you're measuring ~4V across them, then they won't light, but if its say 4 in series (4*3.4V min = 13.6V) and you measure >14V, but the LED's won't light, then one oor all of the LEDs in that string have failed. We can't know without more info