Butane should be purged at minimum 110* F (43*C) and a minimum vacuum of -26HG but the closer to -30HG is optimal for 24-72 hours depending on desired consistency. The extract should also be whipped well several times in the first few hours of the purge. If shatter is the goal, the longer the purge the better.
A cool and fast purge with no vacuum makes butane soup. Not something you want to eat, vape, or smoke. This is why so many people are getting sick from black market BHO. Too many amateurs making the stuff and not purging it off properly or using dirty butane not suitable for extracting
I don't do the vacuum thing, but I do keep it moving as it starts to get sticky, and I'll warm it slightly (hand-hot) to keep it moving and help the bubbles pop. It's always left for a couple of days between extraction and dilution, and putting it in a wide, flat dish over a table lamp warms it up nicely, and provides a large surface area
I do the same thing with alcohol extractions
I would have thought that diluting it with a solvent that isn't miscible with butane would also help to drive off any residual butane.
It may not be perfect, but it's been working for me for quite some time now