Thanks Ethereal_Medic for the answer on Workers.
Not me, it was "cc" that explained the worker-situation.
I can only support the opinion. Early one large vaults are the most cost-efficient solution mixed with large barracks and 1 burrow for most bases.
Once you've 3-4 flourishing production bases (means 3-4 bases entirely focused on producing shit for auto-sell like x-grog or chemicals) you can start to hire workers and replay ALL vaults with them.
Even the slot for an 'armored vault' can be used alot more efficient by placing either a luxury barracks or another workshop on that slot.
Vaultspace can be supported either with improving your slaves (Farmhouse, Supermaid, Drone Herder, Slave Robots in general) and workers.
Having an army of slaves is without doubt the best source of creating 'vaultspace' and each slave-class individually reduces your monthly upkeep to pay the salaries and maintaince costs.
Don't worry, you can reduce your vaultspace to -6000 and more and the game will not crash.