As far as I understand VCs(General Partners) don't hold investment money. They just get paid fees and have a mandate from Limit Partners to invest, so when they want to invest, they just call on LPs to transfer the money.

I imagine each VC fund has a multitude of LPs and they each manage cash based on their own strategies. The key is that they have enough liquid assets to meet the obligations in a timely manner.

Obviously, each fund is set up separately and might have a different structure.