I think if your goal is to quit smoking, you will do the "cut over" when you're ready. Like others have said, you have to want to do it.
Also, there's no one rule or way. But if "paralleling" and "smoking reduction->quitting" doesn't work for you, you'll have to pick a date and cut-over.
It's on you. Sorry. That's the bad news. YOU have to decide. And if one way doesn't work...don't quit quitting. Try another.
I can tell you a few things about it. I can also tell you how I did it. (Been 5 years now)
A few notes first:
1) Realize that by vaping AND smoking, you get COMBINED RISK. If there's any additional risk from vaping that's not in smoking, even if minor, and you're still smoking too....there's combined risks. That's not necessarily great unless your goal is to quit fairly soon. The debate is if the reduced smoking lowers your risks any and 'offsets' any vaping risks. And the vaping risks are a bit undefined. We know it's better/lower, but I don't know of anyone that things it is risk-free.
2) Tobacco has many properties...they work in combination with the nic. By switching to vaping (or any NRT type nic) you don't get the exact same feeling as with smoking. Because it's "just nic".
3) All legitimate indications are that normal vaping is lower risk. So it's worthwhile to pursue quitting smoking. Your instincts are good that you should.
OK. One method that worked for me:
Step 1: Parallel for a while like you're doing. A few weeks maybe. This let's you build your vaping knowledge and gives you a "security blanket" that you won't be without a working system. Get some spares. Make sure whatever devices and juices you are using work for you. Don't be scared of the nic...you don't have to reduce the nic right away.
Step 2: Set a quit-date. For me it was saying "this last carton of cigs are the last cigs I will ever buy." I stuck to it. I paralleled for a while and stretched out the cigs, but the last smoke was the last smoke.
Step 3: Quit. Vape only. Ask any other smokers that live with you if they could please smoke outside for a month.
Around week 2 or 3 was the hardest. It takes a while to detox from tobacco smoking. You may get some TEMPORARY side-effects and withdrawal beyond the nic. If you know what it is ahead of time, you can prepare yourself for it and push though it. It only lasted about a week for me. The worst of the cravings. The quit-zits. The minor sinus issues as my body went back to normal. Like I said, it didn't last long.
The biggest thing for me was not having any around, and not buying any. Get some gum or lozenges too if you're in a place where you can't smoke.
Other tips:
Also note that vaping too much at night can keep you awake and give you "lucid dreams" if you have too much nic at night. So may want lower nic juice at night or reduce vaping just before bed.
Once you get a day under your belt, you'll want to make it two. After two, you "sure don't want to give up now! I've got TWO full days without smoking!" And then three, etc. The success builds up if you let it and count it.
If you trip up, start over. Don't beat yourself up, but don't rationalize it either. If your goal is to quit as you say...quit.
That doesn't mean you cant reduce your smoking first. That's fine if it works for you. For many though, it's hard to "cut over". It has to be a real goal for you.
As a last resort, there are liquids that are WTA juices that have tobacco alkaloids in them. These are some of the "missing things" with normal vaping. That may help if you find you cannot quit without them.
Good luck to you. I hope this helps. Please understand this is just one person's perspective and there's no 100% perfect way that works for everyone. Do what works for you and don't quit quitting.