I used Vyvanse 30-50 mg (ADHD diagnosed) for about a year and quit it 1 month ago to try and see if Modafinil 200 mg is more effective. I also dropped caffeine at the same time.
My reason for dropping Vyvanse wasn't the crash but more because my brain felt exhausted from all the stimulation and I think my ADHD symptoms are more caused by narcolepsy which Modafinil is also indicated for. I dropped caffeine just because, I honestly don't think I have ever gotten a good effect from drinking it and it causes me acute stress if anything.
I have never used Adderall. Vyvanse is supposed to not be addicting like Adderall is. Personally I have taken 1-3 week breaks from Vyvanse a few times and I have never gotten "withdrawals" where I'm fiending for an amphetamine hit. I feel normal when taking a break but as expected my ADHD symptoms do get worse. My sleep is about the same when taking a break.
So far been taking Modalert 200 mg (the most expensive brand out there, people say the crystals are smaller and release better or something), never bothered with 100 mg and I am glad I didn't waste time ramping up because I tolerate it just fine. I feel more wakeful during the day (less yawning, less tiredness overall) and I don't feel that stimulant feeling.
I have had a handful of days where Modafinil felt like it gave me an anti-ADHD effect but Vyvanse wins hands down in that regard. I am judging this based off my ability to start and complete tasks. I have more ADHD symptoms after replacing Vyvanse with Modafinil and by that I mean things like shaking my leg, multitasking uncontrollably, forgetting things, procrastinating.
Important context here is that I recently found out I am probably a chronic insomniac, less than 45 min REM per day and less than 6 hours sleep per night on average. My sleep is marginally better because I have put in effort to improve it. Overall while Modafinil makes me feel more awake I plan to start Vyvanse again since it is more effective for ADHD by far.
My thoughts are that stimulant ADHD meds are insanely effective once you're stable (not feeling burnt out and getting a solid >7 hours of sleep for at least 7 days straight). They make my life more enjoyable but if you're not stable they cause a lot of harm by pushing you further into burnout and insomnia. Narcolepsy greatly worsens my ADHD but it is not the root cause.
In short, Modafinil promotes wakefulness but doesn't work as well for ADHD. It feels cleaner than Vyvanse. Going forward I plan to use both at the same time. I don't recommend using it to replace stimulant ADHD meds but it could be useful for taking a break from stimulants when your brain feels burnt out.