After using Vallejo for a while, and following their recommendation to roll the bottle between your hands for mixing/stirring, once I had emptied a bottle I removed the nozzle and examined what was left behind as residue. The remaining material was a huge amount of solids. Now I take the bottle nozzle off, very easy to do, just push it a bit sideways to lift an edge at the seal, a dull blade or fingernail then will fit under and lift it the rest of the way off. No damage to the nozzle or bottle, remains fully serviceable.
Then I have access to the interior, for a complete stirring with whatever will fit inside and go to the bottom. I can tell how well all of the ingredients are blended, by periodically looking at what is still gathered at the bottom of the stirring stick. Once it's all mixed I put the nozzle and cap back on, then give it a good shaking for several seconds, to complete the process.
If I recently used the bottle, (2 or 3 days,) I do think it's OK to shake for mixing, any longer and I repeat the stick stir method. A DuPont rep once told me, paint is a "system," designed to work as intended ONLY if instructions are fully complied with. The DuPont Imron Polyurethane paint has emphatically clear instructions on the label, "MUST BE THOROUGHLY STIRRED BEFORE APPLICATION. It was clear to me that if you don't get it all right, you're not getting all that you paid for.
If you feel like trying it, shake a bottle of Vallejo well, then remove the nozzle, stir it around for a few turns, check the stick and see what is balled up on the stick end. Vallejo works much better for me when I do a complete stir. Takes a little longer, yes, but I think well worth the time.
Patrick