As @CoyoteOldMan says, VST baskets can be less forgiving, meaning your grind consistency and puck prep need to be perfect otherwise your extractions will highlight any imperfections.
Weirdly I couldn’t get on with VST baskets in the past but have had no problem with the La Marzocco ones. I wonder if the LM ones aren’t quite the same as an aftermarket VST.
Again, not that it really matters in this case.
It’s hard to give a one size fits all answer as there’s a few variables here (regarding the actual dose). Firstly coffee density across beans and roast levels. You’ll notice 18g of one coffee will take up more of the basket than 18g of a different coffee.
Secondly is basket size.
Thirdly (which is a product of the above two) is headroom. Too much headroom and your extractions will be thrown off as the rate of saturation will be different due to water having to fill that space before reaching the puck. Too little and you can fracture the puck (and make a mess of your shower screen).
I can’t say this is exclusively true because I’ve not tried every coffee/machine/basket/dose combination but I’ve never had good results under-dosing a basket, and 99% of my good results have been somewhere around 18-19g in, 36-38g out in a basket designed to hold 17 or 18g.
If you do want to dose less, then use a smaller basket. But I’d always aim to dose above the basket size and not below.
