With quarantine in effect here in Colorado (and I can't get to the gym) I've been doing a fair bit of vest work and wanted to add my thoughts.
First off, a vest isn't a substitute for a heavy ruck with a pack. Even with both of my vests I'm only at 40 lbs which is only half or so of what a heavy pack would be. So particularly on long rucks you're just not getting the same load on your body as you would with a pack.
However, as several of you have noted, 20-40 lbs on a vest feels pretty terrible compared to 50 lbs in a good pack. A vest is extremely taxing on the body because it loads it up near the top (shoulders) vs near the middle (hips). Someone mentioned it causes low back stress and I would heavily disagree, low back stress infers that you're not stabilizing your core which allows your pelvis to rock forwards or backwards which does stress the low back. Its like doing a deadlift with a hunched back or a front squat with an over arched low back, your legs may be able to move the weight but if your core can't stabilize if you're in bad place. If you're experiencing low back stress I would propose you need more core strength and need to work on your posture. I think this is vital for hunting because we aren't walking on a nice flat smooth path, we're off camber in deadfall, we're scrambling over scree slopes and post holing in 2 feet of snow, we need to be able to stabilize our spine in all position and for that a rock solid core is important. Loading up that core for extended periods of time is taxing, and it'll take way more out of you that you might expect, a vest is a good way to work on this long duration stability.
Finally, a vest just sucks, all the way around, it makes bending over hard, it makes breathing hard, it makes moving your arms hard, it makes getting up from the ground hard - and taking a vest off makes you feel like super man. When I'm hunting I'm wearing heavy footwear, I'm wearing a bino harness, I'm chest carrying a firearm, I've got winter layers on because I'm outside for hours in crappy weather. A vest doesn't perfectly replicate all of those things, just as it doesn't replicate a heavy pack, but the level of awful it provides is a pretty good simulation. I'm not sure I would go out and buy one for hunt training, but if you've got one, I'd wear it regularly.