Once you commit capital to a VC fund you generally can't pull out. VCs don't take the money immediately, but eventually they will issue capital calls, which require you, as an LP, to wire them money. Your commitment amount is basically the most money, in total, you're willing to wire them over the course of an entire fund (10 years).

It's important to note that most of the time when VCs do poorly they still find a way to return most of the original investment to the LP. The downside is not nearly as pronounced as the potential upside. So you can invest in several VC funds and if only one wins the math will still work out in your favor.

LPs know what VCs are. LPs have risk spreads and VC is (IIRC) the riskiest legal asset class. They expect to (on average) lose money on VC commitments, but the upside is much higher when a VC hits, meaning a single win can make up for a lot of losses. Picking VCs that win is one way LPs generate outsized returns on their assets.

Depends on the firm, but there is usually a "lock-up" period of several years where you cannot divest. Sometimes the firms will return the money early if things are getting bad but once the money is deployed they can't risk a bank run because they'd have to fire sale their investments which would be bad for everyone.