At 70W you're possibly more looking at dual battery mods to get half decent runtime out it. If you were running that smok at 70W on a single 18650 you'd have booted the f*ck out that battery anyway, are you sure you tried another battery in it? I seem to remember reading it took 21700, maybe I'm imagining that bit.
This is the root cause of the problem you're experiencing.
Hate to say it - but... most Vapcell batteries are shit. They are a rewrap company and what you get for your money is a pretty looking label with dubious performance claims wrapped around a mediocre donor battery. Like you said the mod can't be adjusted for power and it runs at an estimated 70 Watts. 70 Watts is going to run down any 18650 really fast - and in the case of a Vapcell it will kill it pretty quickly, and it now looks like your battery is mullered after only a few months use.
It a mod can take a 21700, then USE one. It may come with an 18650 adaptor, but as you have no choice but running at high power you are better off using a 21700 as it is a stronger battery with higher capacity, so it won't be pushed as hard as an 18650 and will give longer run time.
If you must use an 18650 in that mod - use either a Sonly VTC5a, Molical P26a or Molicel P28a. They are the only commonly available 18650s that are Amp rated high enough to cope with 70 Watt vaping - don't expect a charge to last long though as you will be pushing them pretty hard at 70 Watts, but the battery will give you a lot more charge cycles than a shitty Vapcell before it needs replacing. However a decent 21700 is always the best choice.
I recommend you look at Fogstar.co,uk - either a Molicel P42 or P45 is probably the best choice for you. There is a discount code - POTV20.
Before buying any batteries it is probably wise to ask here first - there is a lot of crap out there, and even vape shops tend to stock crap batteries.