I don't know how anyone can vape something within hours of mixing it. There is no defined flavor unless you're using 15-20% of each flavor concentrate. It's an absolute must. Try Flavorah lemonade day one and again day 30. Without steeping, that flavor is overwhelming to say the least. On the opposite end, try Cappela sweet strawberry day one and again day 30. There is barely a hint day one versus a bold strawberry day 30.
In other aspects, some flavors have a very strong alcohol taste that needs to tone down which can only be done over time. You have to give an adequate amount of time.
It's science. Think of marinating chicken. You don't just make your marinade, stick the chicken in it for 15 minutes and then cook it. Typically you would leave it overnight or maybe 2 night. Why? Because it takes time for that marinade to penetrate the thick, dense tissue of the chicken. The longer you marinate the chicken, more of the flavor is present throughout the entire piece, not just the outside of it. The same concept applies here. VG is thick and viscous and it does not adapt quickly to flavors. Essentially what happens is the flavoring needs to break down the molecules of the VG in order for it to adapt to them. More so in juice with a high VG content. This can't happen instantaneously, it can only take time. And regardless of all the "quick" steeping methods out there, i.e. hot water, ultrasonic cleaner, etc...., it's time that provides the best results.
In closing, steeping is a necessity. Unless you're satisfied with vaping VG with minuscule, bland notes of the flavor concentrates that you mixed in it, it is critical. And all flavors are different. Some may take longer than others, however the results in the end with pay dividends of satisfaction when you taste your final product.