"liquid caffeine" doesn't exist (not at atmospheric pressure anyway.) Caffeine is a solid at room temperature, and sublimes (ie: changes directly from solid to gas, with no intermediate liquid phase,) at a temperature of 178'C.
What these sites are presumably selling will be a solution of caffeine in some other solvent, and given that concentrations above ~5% in any food-safe solvents are problematic (it tends to precipitate out as sludgy crystals, and in extreme cases sets solid like chalk,) I think you'll be hard pressed - once you've added nic base, glycols and flavourings to this dilute caffeine solution - to make a finished juice with anything like as much caffeine in as the caffeinated juices TW sells.
The TWDV caffeinated premixes contain 2.5% caffeine (25mg/ml) and the flavour concentrates contain 5% (50mg/ml)
Maybe they've found a food-safe solvent that allows higher concentrations, but I've been looking for one for a couple of years now, and the only solvents that allow high concentrations I've been able to find are chloroform, dichloromethane, and a few other compounds, all of which are singularly undesirable by inhalation. I would guess they're using ethanol or a relatively non-polar glycol (like PG,) and that the solution is =<5% caffeine w/v (but I've not seen the details, so this is completely guesswork on my part.)