It turns out I've been doing it all wrong: I should have been defining the CPU and RAM settings in the Vagrantfile, as that is where the VM guest will get its resource definitions from.
The settings that I've been configuring in the Packer file are, confusingly, for building the VM image, not running it.
Also, I'm taking this opportunity to bump up the guest RAM to 12 GiB, because it's just a nice, round, clean number. This does mean a reduction of ~10% RAM for the host (25% -> ~15%), so I'm hopeful that that is enough.
It turns out I've been doing it all wrong: I should have been defining the CPU and RAM settings in the Vagrantfile, as that is where the VM guest will get its resource definitions from.
The settings that I've been configuring in the Packer file are, confusingly, for building the VM image, not running it.
Also, I'm taking this opportunity to bump up the guest RAM to 12 GiB, because it's just a nice, round, clean number. This does mean a reduction of ~10% RAM for the host (25% -> ~15%), so I'm hopeful that that is enough.