it's not that hard to calculate:
Watt hours(Wh) = Ampere hours(Ah) * Volts (V)
the stated 20.000 mAh= 20 Ah, the usual li voltage is 3.7 V
so that's 74 Wh capacity in the power bank, it will loose say 20% for the dc-ac conversion, so 60 Wh are avaiable, which should be 1,5h at 40w consumption of the vape by theory.
Wh (battery) / W (consumtion) = time (h)
But no battery likes being discarged completly, so subtract another 20% at least, so it runs maybe 1h, if the 40w are correct...
Another thingy is many heaters draw much more energy at the switching on moment, so that could blow a fuse at the power bank (hopefully just that) at the on switch - or just don't turn on
If u give some more details how u want to use it (portable everywhere, or just a cabin where is no juice) it would be easier to help