For
Disneyland, absolutely. For Disneysea, only if you made some hard choices about things you were willing to miss out on. I ended up purchasing 1 evening ticket for Disneysea to finish out what I couldn't do during the 1 vp day. The 1 vp day + evening ended up being perfect to accomplish everything that I wanted.
Without the VP, I probably would have needed 2 full days for both parks to accomplish the same, especially when factoring my needs regarding accessing rides, breaks, and other factors. This might not be true for everyone if they are able to manage the longer waits or be more flexible with rope drop, time in the park, etc., so you might get different answers depending on people's touring styles and ability. There probably are others on here who will say it's possible to do it all in 1 day - and most likely for them it actually is.
In case you are curious how much 'upcharge' a vp package would cost (in my case the unlimited package):
I pretty much went with the cheapest unlimited VP for my dates (Toy Story 1 night, no extra dining reservations) and stayed in the cheapest nearby non-Disney hotel the night before/after to cut down on cost. I also tracked while there everything that I got through the VP and afterwards added up what all of it would have cost to purchase out of pocket on my dates - I was curious what the 'upcharge' was for the package.
It ended up being $287 USD more than if I had bought each thing individually. So $287 is basically what I paid for the convenience and flexibility. That's quite a lot, but then without it I probably would have needed to spend more days at TDL, so more hotel nights, and less days to spend experiencing other things in Japan. With that factored in, it was actually cheaper to go with the package than add on extra days.
So it really just depends - what all do you actually want to do in the parks, how long do you have in Japan, and how much are you willing to compromise re time, days, cost, etc.
I usually don't splurge for perks or other things, and when I do, I have to plan very carefully to offset the costs. This was one time that for me was definitely worth it. It may or may not be the case for you, as it will depend on what it is you want to get out of your time in the parks, and how much you want to 'see/do it all' vs. be comfortable with possibly/probably missing out on some things if you aren't able to catch the lower waits or get the
lightning lane.