The answer to your question has less to do with "vibe coding," "harness," and all the jargon and "productivity guru speak."

It is akin to asking, "How do you get a runner's high when running in the Nike Vaporfly?" You don't. What you _can_ do is improve your core strength, endurance, cardio, etc. Then, there will come a day, if you are consistent in your running, when, while running, you are less worried about tired legs and generally feel "open" and feel like flying. That day, you will push your pace and realise "oh shit" and bonk. That day you realise that your body and mind are not ready for sustaining a very high pace for a long time. You start doing interval training, hill repeats, and so on. A few months later you try to push the pace again on a good day and voila, you love it! Pure running bliss.

The same is true for engineering. If your "multitasking" muscle/mechanism is weak, if your orbital-prefrontal cortex is weak and you get distracted easily, then no amount of process hacking will get you to flow state. So first understand your mind, understand your body, understand your weaknesses. Work on them consciously, deliberately.

As for me, I can work on at most 2, maybe 3, projects at a time. I observed that my mind works well as a stack, and I am good at pushing context from one project onto the stack, loading the next project's context, then pushing it onto the stack and starting work on the next one. Finish it, then pop to the previous project, and keep going around. When my mind feels weak, I use digital sticky notes or just jot down notes somewhere to maintain context between projects, to just drop down t o focusing on 1 project a time.