overall, this doesn't seem quite right. the difference in the lowest gear on these two bikes ought not to be enough to explain such a dramatic difference.
the gearing on last year's vado 3, and presumably most of them (?) is a 40t chainring with an 11-42 cassette. specialized website has some wierd info, but this is what every review i've seen said. that means the lower gear is a .95 ratio, which is quite low! tires are 700x47mm, or 28.2" outside diamater.
the new vado 5 with IGH supposedly uses the enviolo "heavy duty" IGH with a range from .5 to 1.90. the rear cog is 24, the front is 50. so 50/24*.5 = 1.04. so on the 3.0, one turn of the pedals makes the wheel turn .95 times, and on the 5.0 IGH, it turns 1.04 times. that's really not a huge difference - around 10%. 27.6" outer diameter of the 650b x 2.3" tires shrinks that difference a bit, down to 8%. you'd certainly feel that difference riding but the motor in the 5.0 ought to be able to deal with that no problem at all. it has a ton of torque!
it would be useful to look at what mission control says while you're attempting this hill - are you pedaling hard enough to get the full assist level available? to get the full 560w of power that the vado 5 can provide you need to be providing around 140 yourself ("as much as 4x you" per specialized). perhaps the lower gearing of the old bike had you pedaling a bit faster and harder? again, mission control "rider power" and "motor power" real time statistic would answer the question handily.
that said - a 20% hill is VERY steep. extremely unusual for a paved road, which are typically designed to max out from 6-10 percent in most of the relatively recently built western world. it takes almost 800 watts of power to propel a 260lb object (bike plus rider) up a 20% grade at 7mph.
From what I've noticed, this is likely the issue.. the IGH in the lowest gear, you have to pedal HARD to get the gearing to tighten up and bite like you can with the low gear on a standard derailleur.. and the second you let off of the pedals, the gears lose their grip and will loosen right up. This could actually be pretty dangerous on a hill that steep as well..